Sericin extracted fabrics

ABSTRACT

Silk is purified to eliminate immunogenic components (particularly sericin) and is used to form fabric that is used to form tissue-supporting prosthetic devices for implantation. The fabrics can carry functional groups, drugs, and other biological reagents. Applications include hernia repair, tissue wall reconstruction, and organ support, such as bladder slings. The silk fibers are arranged in parallel and, optionally, intertwined (e.g., twisted) to form a construct; sericin may be extracted at any point during the formation of the fabric, leaving a construct of silk fibroin fibers having excellent tensile strength and other mechanical properties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/561,808, filed Jul. 30, 2012, which is a divisional of U.S. patentapplication Ser. No. 12/770,588, filed Apr. 29, 2010, which is acontinuation of U.S. patent application Ser. No. 10/800,134, filed Mar.11, 2004, which claims the benefit under 35 U.S.C. §119(e) to U.S.Provisional Application No. 60/453,584, filed Mar. 11, 2003, and is acontinuation-in-part of U.S. patent application Ser. No. 10/008,924,filed Nov. 16, 2001, now U.S. Pat. No. 6,902,932, each of which areincorporated herein by reference in their entirety.

BACKGROUND

Disease, aging, trauma or chronic wear often lead to tissue or organfailure. In treating such failures, the goal of many clinical procedureis restoration of function. A patient often requires additional support,beyond the body's own means of healing, such as surgery or theimplantation of a medical device. Such procedures are often needed tocombat permanent disability and even death. The fields of biomaterialsand tissue engineering are providing new options to gradually restorenative tissue and organ function through the research and development oftemporary scaffolds, matrices, and constructs (i.e., devices) thatinitially support a disabled tissue or organ, but eventually allow forthe development and remodeling of the body's own biologically andmechanically functional tissue.

The responsibilities or design requirements of such a scaffold include:(i) the ability to provide immediate mechanical stabilization to thedamaged or diseased tissue, (ii) support cell and tissue ingrowth intothe device, (iii) communicate the mechanical environment of the body tothe developing tissue; such is achieved through the proper mechanicaland biological design of the device, (iv) degrade at such a rate thatthe ingrowing cells and tissue have sufficient time to remodel, thuscreating new autologous function tissue that can survive the life of thepatient. In certain instances, the device should mimic the correctthree-dimensional structure (e.g., a bone scaffold) of the tissue it isattempting to support. In other instances, the device may serve as atemporary ligature (e.g., a flat mesh for hernia repair or a hemostatfor bleeding) to a three-dimensional tissue (abdominal wall muscle inthe case of hernia). Regardless of application, the present direction ofthe medical device field is the complete restoration of bodily functionthrough the support of autologous tissue development.

Unfortunately, most biomaterials available today do not possess themechanical integrity of high load demand applications (e.g., bone,ligaments, tendons, muscle) or the appropriate biological functionality;most biomaterials either degrade too rapidly (e.g., collagen, PLA, PGA,or related copolymers) or are non-degradable (e.g., polyesters, metal),where in either case, functional autologous tissue fails to develop andthe patient suffers disability. In certain instances a biomaterial maymisdirect tissue differentiation and development (e.g., spontaneous boneformation, tumors) because it lacks biocompatibility with surroundingcells and tissue. As well, a biomaterial that fails to degrade typicallyis associated with chronic inflammation, where such a response isactually detrimental to (i.e., weakens) surrounding tissue.

If properly designed, silk may offer new clinical options for the designof a new class of medical devices, scaffolds and matrices. Silk has beenshown to have the highest strength of any natural fiber, and rivals themechanical properties of synthetic high performance fibers. Silks arealso stable at high physiological temperatures and in a wide range ofpH, and are insoluble in most aqueous and organic solvents. Silk is aprotein, rather than a synthetic polymer, and degradation products(e.g., peptides, amino acids) are biocompatible. Silk is non-mammalianderived and carries far less bioburden than other comparable naturalbiomaterials (e.g., bovine or porcine derived collagen).

Silk, as the term is generally known in the art, means a filamentousfiber product secreted by an organism such as a silkworm or spider.Silks produced from insects, namely (i) Bombyx mori silkworms, and (ii)the glands of spiders, typically Nephila clavipes, are the most oftenstudied forms of the material; however, hundreds to thousands of naturalvariants of silk exist in nature. Fibroin is produced and secreted by asilkworm's two silk glands. As fibroin leaves the glands, it is coatedwith sericin, a glue-like substance. However, spider silk is valued (anddifferentiated from silkworm silk) as it is produced as a singlefilament lacking any immunogenic contaminates, such as sericin.

Unfortunately, spider silk can not be mass produced due to the inabilityto domesticate spiders; however, spider silk, as well as other silks canbe cloned and recombinantly produced, but with extremely varyingresults. Often, these processes introduce bioburdens, are costly, cannotyield material in significant quantities, result in highly variablematerial properties, and are neither tightly controlled norreproducible.

As a result, only silkworm silk has been used in biomedical applicationsfor over 1,000 years. The Bombyx mori specie of silkworm produces a silkfiber (known as a “bave”) and uses the fiber to build its cocoon. Thebave, as produced, includes two fibroin filaments or “broins”, which aresurrounded with a coating of gum, known as sericin—the silk fibroinfilament possesses significant mechanical integrity. When silk fibersare harvested for producing yarns or textiles, including sutures, aplurality of fibers can be aligned together, and the sericin ispartially dissolved and then resolidified to create a larger silk fiberstructure having more than two broins mutually embedded in a sericincoating.

As used herein, “fibroin” includes silkworm fibroin (i.e. from Bombyxmori) and fibroin-like fibers obtained from spiders (i.e. from Nephilaclavipes). Alternatively, silk protein suitable for use in the presentinvention can be obtained from a solution containing a geneticallyengineered silk, such as from bacteria, yeast, mammalian cells,transgenic animals or transgenic plants. See, for example, WO 97/08315and U.S. Pat. No. 5,245,012.

Silkworm silk fibers, traditionally available on the commercial marketfor textile and suture applications are often “degummed” and consist ofmultiple broins plied together to form a larger single multi-filamentfiber. Degumming here refers to the loosening of the sericin coatsurrounding the two broins through washing or extraction in hot soapywater. Such loosening allows for the plying of broins to create largermultifilament single fibers. However, complete extraction is oftenneither attained nor desired. Degummed silk often contains or isrecoated with sericin and/or sericin impurities are introduced duringplying in order to congeal the multifilament single fiber. The sericincoat protects the frail fibroin filaments (only ˜5 microns in diameter)from fraying during traditional textile applications wherehigh-through-put processing is required. Therefore, degummed silk,unless explicitly stated as sericin-free, typically contain 10-26% (byweight) sericin (see Tables 1 & 2).

When typically referring to “silk” in the literature, it is inferredthat the remarks are focused to the naturally-occurring and onlyavailable “silk” (i.e., sericin-coated fibroin fibers) which have beenused for centuries in textiles and medicine. Medical grade silkworm silkis traditionally used in only two forms: (i) as virgin silk suture,where the sericin has not been removed, and (ii) the traditional morepopular silk suture, or commonly referred to as black braided silksuture, where the sericin has been completely removed, but replaced witha wax or silicone coating to provide a barrier between the silk fibroinand the body tissue and cells. Presently, the only medical applicationfor which silk is still used is in suture ligation, particularly becausesilk is still valued for it mechanical properties in surgery (e.g., knotstrength and handlability).

Despite virgin silk's use as a suture material for thousands of years,the advent of new biomaterials (collagen, synthetics) have allowed forcomparisons between materials and have identified problems with sericin.Silk, or more clearly defined as Bombyx mori silkworm silk, isnon-biocompatible. Sericin is antigenic and elicits a strong immune,allergic or hyper-T-cell type (versus the normal mild “foreign body”response) response. Sericin may be removed (washed/extracted) from silkfibroin; however, removal of sericin from silk changes theultrastructure of the fibroin fibers, exposing them, and results in lossof mechanical strength, leading to a fragile structure.

Extracted silk structures (i.e., yarns, matrices) are especiallysusceptible to fraying and mechanical failure during standard textileprocedures due to the multifilament nature of the smaller diameter (˜5um) fibroin filaments. The extracted fibroin's fragility is the reasonthat when using silk in the design and development of medical devices,following extraction, it is typically taught (Perez-Rigueiro, J. Appl.Polymer Science, 70, 2439-2447, 1998) that you must dissolve andreconstitute silk using standard methods (U.S. Pat. No. 5,252,285) togain a workable biomaterial. The inability to handle extracted silkfibroin with present-day textile methods and machinery has prevented theuse of non-dissolved sericin-free fibroin from being explored as amedical device.

Additional limitations of silk fibroin, whether extracted from silkwormsilk, dissolved and reconstituted, or produced from spiders or insectsother than silkworms include (i) the hydrophobic nature of silk, adirect result of the beta-sheet crystal conformation of the core fibroinprotein which gives silk its strength, (ii) the lack of cell bindingdomains typically found in mammalian extracellular matrix proteins(e.g., the peptide sequence RGD), and (iii) silk fibroin's smoothsurface. As a result, cells (e.g., macrophages, neutrophils) associatedwith an inflammatory and host tissue response are unable to recognizethe silk fibroin as a material capable of degradation. These cells thusopt to encapsulate and wall off the foreign body (see FIG. 18A) therebylimiting (i) silk fibroin degradation, (ii) tissue ingrowth, and (iii)tissue remodeling. Thus, silk fibroin filaments frequently induce astrong foreign body response (FBR) that is associated with chronicinflammation, a peripheral granuloma and scar encapsulation (FIG. 18A).

In addition to the biological disadvantages of silk, the multifilamentnature of silk (e.g., as sutures) as well as the small size of thefibroin filaments can lead to a tightly packed structure. As such, silkmay degrade too rapidly. Proteases (enzymes) produced from thestimulated cells found within the peripheral encapsulation can penetratethe implanted structure (see FIG. 11A and FIG. 11B), but cellsdepositing new tissue (e.g., fibroblasts) which may reinforce the device(in this case a black braided suture) during normal tissue remodelingcannot. Therefore, the interior of non-treated or non-modified fibroindevices does not come in contact with the host foreign body response andtissue (led and produced by fibroblasts) and as a result, the capacityof the device to direct tissue remodeling is limited. Host cell andtissue growth is limited and degradation is not normally possible.

In the case of sutures, it is thought that these problems can be managedby treating fibroin sutures with cross-linking agents or by coating thesutures with wax, silicone or synthetic polymers, thereby shielding thematerial from the body. Coatings, such as sericin, wax or silicone,designed to add mechanical stability to the fibroin (combating itsfragility while providing a barrier between the body and the fibroin),limits cell attachment, recognition and infiltration and tissue ingrowthand fibroin degradation. As a result, silk is traditionally thought ofas a non-degradable material.

Classification as a non-degradable may be desirable when silk isintended for use as a traditional suture ligation device, i.e., cell andtissue ingrowth into the device are not desirable. Therefore, cellattachment and ingrowth (which lead to matrix degradation and activetissue remodeling) is traditionally prevented by both the biologicalnature of silk and the structure's mechanical design. In fact, a generalbelief that silk must be shielded from the immune system and theperception that silk is non-biodegradable have limited silk's use insurgery. Even in the field of sutures, silk has been displaced in mostapplications by synthetic materials, whether biodegradable or permanent.

Therefore, there exists a need to generate sericin-extracted silkwormfibroin fibers that are biocompatible, promote ingrowth of cells, andare biodegradable.

SUMMARY

Natural silk fibroin fiber constructs, disclosed herein, offer acombination of high strength, extended fatigue life, and stiffness andelongation at break properties that closely match those of biologicaltissues. The fibers in the construct are non-randomly aligned into oneor more yarns. The fiber constructs are biocompatible (due to theextraction of sericin from the silkworm silk fibers) and substantiallyfree of sericin. The fiber constructs are further non-immunogenic; i.e.,they do not elicit a substantial allergic, antigenic, or hyper T-cellresponse from the host, diminishing the injurious effect on surroundingbiological tissues, such as those that can accompany immune-systemresponses in other contexts. In addition, the fiber constructs promotethe ingrowth of cells around said fibroin fibers and are biodegradable.

Indications that the fiber construct is “substantially free” of sericinmean that sericin comprises less than 20% sericin by weight. Preferably,sericin comprises less than 10% sericin by weight. Most preferably,sericin comprises less than 1% sericin by weight (see Table 2).Furthermore, “substantially free” of sericin can be functionally definedas a sericin content that does not elicit a substantial allergic,antigenic, or hyper T-cell response from the host. Likewise, indicationthat there is less than a 3% change in mass after a second extractionwould imply that the first extraction “substantially removed” sericinfrom the construct and that the resulting construct was “substantiallyfree” of sericin following the first extraction (see Table 2 and FIG.1F).

Methods of this disclosure extract sericin from the construct much morethoroughly than do the typical “degumming” procedures that characterizetraditional processing practices for the production of silk textiles fornon-surgical applications (see above for definition). FIG. 1A shows animage of a degummed fiber where fibroin filaments were plied togetherforming a larger fiber re-encased with sericin. This “degummed” fibercontains ˜26%, by weight, sericin. In a preferred embodiment, thesericin-extracted silkworm fibroin fibers retain their native proteinstructure and have not been dissolved and reconstituted.

“Natural” silk fibroin fibers are produced by an insect, such as asilkworm or a spider and possess their native, as formed, proteinstructure. Preferably, the silk fibroin fiber constructs arenon-recombinant (i.e., not genetically engineered) and have not beendissolved and reconstituted. In a preferred embodiment, thesericin-extracted fibroin fibers comprised fibroin fibers obtained fromBombyx mori silkworm. Further, the term, “biodegradable,” is used hereinto mean that the fibers are degraded within one year when in continuouscontact with a bodily tissue. In addition, our data suggests (FIG. 13A-E, FIG. 18 A-C & FIG. 19 A-D) that the rate of degradation can beinfluenced and enhanced by surface modification of the fibroin (FIG. 13A-D & FIG. 18 A-C) as well as the geometric configuration of the yarnand/or fabric (FIG. 19A-D). In one embodiment, silk fibroin yarn lost50% of its ultimate tensile strength within two weeks followingimplantation in vivo (FIG. 12) and 50% of its mass within approximately30 to 90 days in vivo, depending on implantation sight (FIG. 13 A-D).The choice of implantation site in vivo (e.g., intra-muscular versussubcutaneous) was shown to significantly influence the rate ofdegradation (FIG. 13 A-D).

Textile-grade silk” is naturally occurring silk that includes a sericincoating of greater than 19%-28% by weight of the fiber. “Suture silk” issilk that either contains sericin (“virgin silk suture”) or is coatedwith a hydrophobic composition, such as bee's wax, paraffin wax,silicon, or a synthetic polymeric coating (“black braided silk suture”).The hydrophobic composition repels cells or inhibits cells fromattaching to the coated fiber. Black braided silk is a suture silk inwhich sericin has been extracted and replaced with additional coating.Suture silk is typically non-biodegradable.

Due to the absence of a protective wax or other hydrophobic coating onthe fibers the silk fibroin constructs described are biologically(coupling of cell binding domains) and/or mechanically (increase silksurface area and decrease packing density) designed to promote increasedcell infiltration compared to textile-grade silk or suture silk whenimplanted in bodily tissue. As a result, the silk fibroin constructssupport cell ingrowth/infiltration and improved cell attachment andspreading, which leads to the degradation of the silk fibroin constructthereby essentially creating a new biodegradable biomaterial for use inmedical device and tissue engineering applications. The ability of thefiber construct to support cell attachment and cell and tissueingrowth/infiltration into the construct, which in return supportsdegradation, may be further enhanced through fibroin surfacemodification (peptide coupling using RGD, chemical species modificationand increasing hydrophilicity through gas plasma treatment) and/or themechanical design of the construct thereby increasing material surfacearea thus increasing its susceptibility to those cells and enzymes thatpossess the ability to degrade silk. The silk fibers are optionallycoated with a hydrophilic composition, e.g., collagen or a peptidecomposition, or mechanically combined with a biomaterial that supportscell and tissue ingrowth to form a composite structure. The choice ofbiomaterial, amount and mechanical interaction (e.g., wrapped or braidedabout a core of silk fibroin) can be used to alter and/or improve ratesof cell ingrowth and construct degradation.

Fibers in the construct are non-randomly aligned with one another intoone or more yarns. Such a structure can be in a parallel, braided,textured, or helically-organized (twisted, cabled (e.g., a wire-rope))arrangement to form a yarn. A yarn may be defined as consisting of atleast one fibroin fiber. Preferably, a yarn consists of at least threealigned fibroin fibers. A yarn is an assembly of fibers twisted orotherwise held together in a continuous strand. An almost infinitenumber of yarns may be generated through the various means of producingand combining fibers. A silk fiber is described above; however, the termfiber is a generic term indicating that the structure has a length 100times greater than its diameter.

When the fibers are twisted or otherwise intertwined to form a yarn,they are twisted/intertwined enough to essentially lock in the relativefiber positions and remove slack but not so much as to plasticallydeform the fibers (i.e., does not exceed the material's yield point),which compromises their fatigue life (i.e., reduces the number of stresscycles before failure). The sericin-free fibroin fiber constructs canhave a dry ultimate tensile strength (UTS) of at least 0.52 N/fiber(Table 1, 4), and a stiffness between about 0.27 and about 0.5 N/mm perfiber. Depending on fiber organization and hierarchy, we have shown thatfibroin construct UTSs can range from 0.52 N/fiber to about 0.9N/fiber.Fibroin constructs described here retained about 80% of their dry UTSand about 38% of their dry stiffness, when tested wet (Table 5).Elongations at break between about 10% and about 50% were typical forfibroin constructs tested in both dry and wet states. Fibroin constructstypically yielded at about 40 to 50% of their UTS and had a fatigue lifeof at least 1 million cycles at a load of about 20% of the yarnsultimate tensile strength.

In one embodiment of the present invention, the alignedsericin-extracted silkworm fibroin fibers are twisted about each otherat 0 to 11.8 twists per cm (see Table 6 & 7).

The number of hierarchies in the geometrical structure of the fiberconstruct as well as the number of fibers/groups/bundles/strands/cordswithin a hierarchical level, the manner of intertwining at the differentlevels, the number of levels and the number of fibers in each level canall be varied to change the mechanical properties of the fiber construct(i.e., yarn) and therefore, fabric (Table 4 & 8). In one embodiment ofthe present invention, the fiber construct (i.e. yarn) is organized in asingle-level hierarchical organization, said single-level hierarchicalorganization comprising a group of parallel or intertwined yarns.Alternatively, the fiber construct (i.e. yarn) organized in a two-levelhierarchical organization, said two-level hierarchical organizationcomprising a bundle of intertwined groups. In another embodiment of thepresent invention, the fiber construct (i.e. yarn) is organized into athree-level hierarchical organization, said three-level hierarchicalorganization comprising a strand of intertwined bundles. Finally,another embodiment of the present invention, the fiber construct (i.e.yarn) is organized into a four-level hierarchical organization, saidfour-level hierarchical organization comprising a cord of intertwinedstrands.

The sericin can be removed from the fibroin fibers before the alignmentinto a yarn or at a higher level in the hierarchical geometry of thefiber construct. The yarn is handled at low tension (i.e., the forceapplied to the construct will never exceed the material's yield pointduring any processing step) and with general care and gentleness afterthe sericin is removed. Processing equipment is likewise configured toreduce abrasiveness and sharp angles in the guide fixtures that contactand direct the yarn during processing to protect the fragile fibroinfibers from damage; extraction residence times of 1 hour are sufficientto extract sericin but slow enough as not to damage the exposedfilaments. Interestingly, when a silk fiber construct consisting ofmultiple fibers organized in parallel has been extracted under theseconditions, a “single” larger sericin free yarn resulted (i.e.,individual fibers cannot be separated back out of the construct due tothe mechanical interaction between the smaller fibroin filaments onceexposed during extraction). Furthermore, as a result of the mechanicalinterplay between the sericin-free micro filaments, extraction oftwisted or cabled yarns has typically resulted in less “lively” yarnsand structures. As a result of this phenomenon, a greater degree offlexibility existed in the design of the yarns and resulting fabrics;for example, higher twist per inch (TPI) levels can be used, which wouldnormally create lively yarns that would be difficult to form intofabrics. The added benefit of higher TPIs was the reduction in yarn andfabric stiffness (i.e. matrix elasticity can be increased)(Tables 6 and7; FIG. 6A and FIG. 6B).

A plurality of yarns are intertwined to form a fabric. Fabrics aregenerated through the uniting of one or more individual yarns wherebythe individual yarns are transformed into textile and medical devicefabrics. In one embodiment of the present invention, the yarn is twistedat or below 30 twists per inch. Fabrics are produced or formed bynon-randomly combining yarns: weaving, knitting, or stitch bonding toproduce completed fabrics. In one embodiment, this combining of yarns toform a fabric is done on a machine. However, it is very important tonote that the end fabric product is distinct based on the yarn type usedto make it thus providing tremendous power through yarn design to meetclinical needs. A fabric can be, but is not limited to, woven, knit,warp-knit, bonded, coated, dobby, laminated, mesh, or combinationsthereof.

Of note, the textile methods of braiding, in addition to making yarns,can also be used to make fabrics, such as a flat braided fabric or alarger circular braid (FIG. 4A). Inversely, weaving and knitting, twofabric forming methods, although not commonly used, can also be used tomake yarns. In such instances, the differentiation between a “yarn” anda “fabric” is not entirely apparent, and the homogeneity should be usedto make clear distinctions, i.e., a yarn is typically more homogeneousin composition and structure than a fabric.

In one embodiment of the present invention, multiple silkworm silkfibers may be organized helically (e.g., twisted or cabled) or inparallel, in a single hierarchical level or in multiple levels,extracted, and used to create a braided suture for tissue ligation. Inanother embodiment, the mechanical interaction of extracted fibroinfilaments in a twisted or cabled configuration following extraction canbe used as a medical suture.

Non-woven fabrics may be formed by randomly organizing a plurality ofyarns, or a single yarn cut into many small length pieces. Non-limitingexamples include a fabric for hemostat or bone scaffold. All fabrics caneither derive from a single yarn construct (homogenous) or multipleyarns constructs (heterogeneous). The ability to design for a variety ofsilk fibroin yarn structures, as described in detail below, dramaticallyincreases fabric design potential when considering a heterogeneousfabric structure.

In one embodiment of the present invention, the fabric is a composite ofthe sericin-extracted fibroin fibers or yarns and one or more degradablepolymers selected from the group consisting of Collagens, Polylacticacid or its copolymers, Polyglycolic acid or its copolymers,Polyanhydrides, Elastin, Glycosamino glycans, and Polysaccharides.Furthermore, the fabric of the present invention may be modified tocomprise a drug associated or a cell-attachment factor associated withfabric (i.e. RGD). In one embodiment of the present invention, thefabric is treated with gas plasma or seeded with biological cells.

Additional aspects of this disclosure relate to the repair of specificbodily tissues, such as hernia repair, urinary bladder tissues andslings, pelvic floor reconstruction, peritoneal wall tissues, vessels(e.g., arteries), muscle tissue (abdominal smooth muscle, cardiac),hemostats, and ligaments and tendons of the knee and/or shoulder as wellas other frequently damaged structures due to trauma or chronic wear.Examples of ligaments or tendons that can be produced include anteriorcruciate ligaments, posterior cruciate ligaments, rotator cuff tendons,medial collateral ligaments of the elbow and knee, flexor tendons of thehand, lateral ligaments of the ankle and tendons and ligaments of thejaw or temporomandibular joint. Other tissues that may be produced bymethods of this disclosure include cartilage (both articular andmeniscal), bone, skin, blood vessels, stents for vessel support and/orrepair, and general soft connective tissue.

In other aspects, silkworm fibroin fibers, in the form of a yarn or of alarger construct of yarns, now termed a device, is stripped of sericin,and made (e.g., woven, knitted, non-woven wet laid, braided, stitchbonded, etc.) into a fabric, sterilized and used as an implantablesupporting or repair material that offers a controllable lifetime (i.e.,degradation rate) and a controllable degree of collagen and/orextracellular matrix deposition. The support or repair material can beused for any such purpose in the body, and in particular can be used forhernia repair, reconstruction of body walls, particularly in the thoraxand abdominal cavity, and support, positioning or immobilization ofinternal organs, including, without limitation, the bladder, the uterus,the intestines, the urethra, and ureters. Alternatively, silkwormfibroin fibers may be stripped of sericin and organized into a non-wovenfabric. Such non-woven fabric can be used as an implantable supportingor repair material as above, but more specifically for applicationswhere a sponge formation would be useful.

The purified silk can be purified by any of a variety of treatments thatremove the sericin proteins found in the native fibrils. Sericin hasbeen removed sufficiently when implants of purified silk elicit only amild, transient foreign body reaction in the absence of an antigenic(B-cell, T-cell) response, i.e., are biocompatible. A foreign bodyreaction is characterized by an inner layer of macrophages and/or giantcells with a secondary zone of fibroblasts and connective tissue. Thedegree of foreign body response has been shown to be controllablethrough fibroin modification (FIG. 13 A-D & FIG. 18 A-C) and yarn design(FIG. 19 A-D). Sericin can be removed from individual silkworm fibroinfibers, a group of silkworm fibroin fibers (i.e. a yarn), having anorganized orientation (e.g., parallel or twisted), or form a fabric orother construct comprising a plurality of yarns. The construct can thenbe sterilized and implanted in an organism as a medical device.

Other features and advantages of the invention will be apparent from thefollowing description of preferred embodiments thereof.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a scanning electron microscopy (SEM) image of a single nativedegummed and plied 20/22 denier silk fiber having a sericin coating.

FIG. 1B illustrates SEM of the silk fiber of FIG. 1A extracted for 60min at 37° C.

FIG. 1C illustrates SEM of the silk fiber of FIG. 1A extracted for 60min at 90° C. and illustrating complete removal of the sericin coating.

FIG. 1D is a chart showing ultimate tensile strength (UTS) and stiffness(N/mm for a 3 cm length matrix) as a function of extraction conditions.

FIG. 1E illustrates SEM of a raw silk fibroin. FIG. 1F illustrates afirst extraction at 90° C. for 60 min. FIG. 1G illustrates a secondextraction under identical conditions. These figures show mechanicaldamage to the filaments that results in a typical 3% mass loss followingthe second extraction. Therefore, as long as the % mass loss does notchange more than 3% from the first to the second extraction (90° C., 1hr, standard detergent and salt), it is assumed that complete extractionhas been achieved. The utility of a 3% loss in total mass loss reflectsthe variability in the measurements, assays and mechanical damageresulting in mass loss of the yarn following the second extraction.

FIG. 2A is a representative 3-D model of a (cable or twisted) yarndepicting its 5 levels of hierarchy (single fiber level not shown).Depending on the number of fibers used in each level, the cord couldserve as either a yarn for knitting a hernia repair mesh or as a cord tobe used in parallel with other cords to form an ACL matrix.

FIG. 2B is a schematic depicting the generation of a two-levelhierarchical twisted or cabled yarn containing 36 fibers before beingplied in parallel to form an ACL matrix or used to generate a weave orknit fabric for tissue engineering and tissue repair (e.g. hernia mesh).The schematic representations visually define two very popular forms offabric formations: a “weave” and a “knit.”

FIG. 2C illustrates a single cord of yarn having a geometry that ishelically organized about a central axis and composed of two levels oftwisting hierarchy. When six cords are used in parallel (e.g., Matrix1), the yarn has mechanical properties similar to a native ligament.

FIG. 2D illustrates a single cord of yarn having a geometry that ishelically organized about a central axis and composed of three levels oftwisting hierarchy. When six cords are used in parallel (e.g., Matrix2), the matrix has mechanical properties similar to a native ligament.

FIG. 3A illustrates load-elongation curves for five samples (n=5) ofMatrix 1 formed from six parallel silk fibroin cords illustrated in FIG.2A.

FIG. 3B is a chart of cycles to failure at UTS, 1680 N, and 1200 N loads(n=5 for each load) illustrating Matrix 1 fatigue data. Regressionanalysis of Matrix 1 fatigue data, when extrapolated to physiologicalload levels (400 N) to predict number of cycles to failure in vivo,indicates a matrix life of 3.3 million cycles.

FIG. 3C illustrates load-elongation curves for three samples (n=3) ofMatrix 2 (n=3) formed from six parallel silk fibroin cords asillustrated in FIG. 2B.

FIG. 3D is a chart of cycles to failure at UTS, 2280 N, 2100 N and 1800N loads (n=3 for each load) illustrating Matrix 2 fatigue data.Regression analysis of Matrix 2 fatigue data, when extrapolated tophysiological load levels (400 N) to predict number of cycles to failurein vivo, indicates a matrix life of greater than 10 million cycles.

FIG. 4A shows images of multiple yarn and fabric forms generated in ourlaboratories. Several different yarn structures, including various typesof braids (i, ii, iv), a flat braid (iii), a varying diameter or taperbraid (v), a larger (˜250 fibers) cabled two-level bundle (vi), aparallel plied and bonded (swaged) yarn consisting 24-12-fiber texturedyarns (vii), a variety of twisted yarns (viii-xi), and a parallel pliedand bonded (swaged) yarn consisting 24-12-fiber two level cabled yarns(xii).

FIG. 4B is a chart of load-elongation curves for (I) a braid (48 fibers,a 4 carrier braider using twisted extracted 12 fiber yarn) and texturedyarns (48 fibers total) and (II) twisted compared to cabled yarns, 12fibers in total—all samples were 3 cm in length.

FIG. 4C is a chart of fatigue data for small yarns, 3 cm in length, ascompared to 3B and 3D for (I) a small cable of 36 fibers and (II) asmall textured yarn of 60 fibers).

FIG. 5A provides strength and stiffness data for a 36 fiber yarn as afunction of 6 different strain rates at which they were tested (N=5 pergroup).

FIG. 5B shows load-elongation curves for a 36-fiber yarn, 3 cm long,tested at 2 of the 6 different strain rates. The data represents theeffect of the testing procedures (here, specifically strain rate) on thereported mechanical properties (e.g. UTS) of the yarn structure.

FIG. 6A is a chart of UTS as a function of twists per inch (TPI); trendlines were generated to extrapolate data to a 4^(th) orderpolynomial—TPIs from 0-15 are shown. A maximum was observed indicatingan ordered structure where individual filaments are working in unison.

FIG. 6B is a chart of stiffness (for a 3 cm length sample) as a functionof twists per inch (TPI); trend lines were generated to extrapolate datato a 5^(th) order polynomial—TPIs from 0-15 are shown. A maximum wasobserved indicating that TPI could be used as a tool to design for aspecific UTS or stiffness.

FIG. 7A illustrates SEM of extracted silk fibroin prior to seeding withcells.

FIG. 7B illustrates SEM of bone marrow stromal cells seeded and attachedon silk fibroin immediately post seeding.

FIG. 7C illustrates SEM of bone marrow cells attached and spread on silkfibroin 1 day post seeding.

FIG. 7D illustrates SEM of bone marrow stromal cells seeded on silkfibroin 14 days post seeding forming an intact cell-extracellular matrixsheet.

FIG. 8A illustrates a 3 cm length of the silk fibroin cord illustratedin FIG. 2C and seeded with bone marrow stromal cells, cultured for 14days in a static environment and stained with MTT to show even cellcoverage of the matrix following the growth period.

FIG. 8B illustrates a control strand of silk fibroin cord 3 cm in lengthstained with MTT.

FIG. 9A is a chart illustrating bone marrow stromal cell proliferationon silk fibroin Matrix 1 determined by total cellular DNA over 21 dayculture period indicating a significant increase in cell proliferationafter 21 days of culture.

FIG. 9B is a bar graph illustrating bone marrow stromal cellproliferation on silk fibroin Matrix 2 determined by total cellular DNAover 14 day culture period indicating a significant increase in cellproliferation after 14 days of culture.

FIG. 10 illustrates the ultimate tensile strength of a 30 silk fiberextracted construct that is either seeded with bone marrow stromal cellsor non-seeded over 21 days of culture in physiological growthconditions.

FIG. 11A is a chart of UTS as a function of in vitro enzymaticdegradation; no strength loss was observed in the negative control, PBS.Silk lost 50% of its strength after 21 days in culture. A 1 mg/mlsolution of Protease XIV from Sigma was used.

FIG. 11B is a chart of mass loss as a function of in vitro enzymaticdegradation; no strength loss was observed in the negative control, PBS.50% mass loss was observed after 41 days in culture.

FIG. 12 is a chart of UTS loss as function of in vivo degradationfollowing RGD-modified matrix implantation into a non-loadedsubcutaneous rat model for 10, 20 and 30 days. 50% strength loss wasobserved after ˜10 days in vivo in a non-loaded environment.

FIG. 13A shows histological sections of 12(0)×3(8) non-modified andRGD-modified sericin-free silk fibroin matrices after 30 days ofsubcutaneous implantation in a Lewis rat. Row I is H&E staining at 40×,row II is H&E staining at 128×, row III is collagen trichrome stainingat 128×, row IV is collagen backed out of the row III images to allowfor collagen ingrowth quantification and row V are the pixels associatedwith the cross-sections of remaining silk fibroins backed out to allowfor quantification of degradation. Upon qualitative assessment, in thesubcutaneous environment, both the non-treated and modified groupssupported cell ingrowth and collagen deposition within the matrix itselfwith limited peripheral encapsulation.

FIG. 13B quantitatively represents a 36% decrease in RGD-modified silkcross-sectional area after 30 days of subcutaneous implantationindicating a significant improvement in the ability of the host todegrade the surface modified silk fibroin matrices compared tonon-treated controls.

FIG. 13C quantitatively shows a significant 63% increase in collagendeposition within the RGD-modified fibroin matrices as compared to thenon-treated controls again demonstrating the ability of the modifiedsilk matrix to support host cell and tissue ingrowth.

FIG. 13D shows H&E staining of an extracted 36 fiber fibroin yarnimplanted intramuscularly in the abdominal was of a Lewis rat. Imagesare shown at 40× and 128× for both non-modified and RGD-modifiedmatrices. Results show, qualitatively, that RGD-modificationdramatically increased cell and tissue infiltration within 30 days invivo. Unlike black braided silk suture or virgin silk suture, noperipheral encapsulation or plasma cells were observed. Compared to thesubcutaneous implants, little to no cell infiltration and collagendeposition was observed in the non-treated controls indicating theeffect of implantation site in addition to surface modification.

FIG. 13E is a numerical representation of mass loss in vivo from the twodifferent modification groups compared to non-treated controls. RGDmodification, followed by gas plasma modification significantly (p<0.05)increased the extent of degradation after 90 days of intra-muscularimplantation. However, it appears degradation was more aggressive in thesubcutaneous environment as compared to the intra-muscular environment,as was expected.

FIG. 14 illustrates gel electrophoretic analysis of RT-PCR amplificationof selected markers over time. The gel shows upregulation in bothcollagen types I and III expression levels normalized to thehousekeeping gene, GAPDH by bone marrow stromal cell grown on Matrix 2over 14 days in culture. Collagen type II (as a marker for cartilage)and bone sialoprotein (as a marker of bone tissue formation) were notdetected indicating a ligament specific differentiation response by theBMSCs when cultured with Matrix 2.

FIG. 15A and FIG. 15B illustrates a single cord of Matrix 1 (not seededat the time of implantation) following six weeks of implantation in vivoand used to reconstruct the medial collateral ligament (MCL) in a rabbitmodel. FIG. 15A shows Matrix 1 fibroin fibers surrounded by progenitorhost cells and tissue ingrowth into the matrix and around the individualfibroin fibers visualized by hematoxylin and eosin staining FIG. 15Bshows collagenous tissue ingrowth into the matrix and around theindividual fibroin fibers visualized by trichrome staining.

FIGS. 16A, 16B and 16C illustrate bone marrow stromal cells seeded andgrown on collagen fibers for 1 day (FIG. 16A) and 21 days (FIG. 16B);RT-PCR (FIG. 16C) and gel electrophoretic analysis of collagen I and IIIexpression vs. the housekeeping gene GAPDH: a=Collagen I, day 14;b=Collagen I, day 18; c=Collagen III, day 14; d=Collagen III, day 18;e=GAPDH, day 14; f=GAPDH, day 18. Collagen type II (as a marker forcartilage) and bone sialoprotein (as a marker of bone tissue formation)were not detected indicating a ligament specific differentiationresponse.

FIG. 17 illustrates real-time quantitative RT-PCR at 14 days thatyielded a transcript ratio of collagen I to collagen III, normalized toGAPDH, of 8.9:1.

FIG. 18A and FIG. 18B are H&E stained cross-sections of 6 bundles of (A)2-0 black braided silk suture and (B) RGD-surfaced modified silk (36fibers/bundle), respectively, 30 days following intra-muscularimplantation. 18C is RGD-modified silk pre-seeded with BMSCs for 4 weeksprior to implantation. FIG. 18A shows a typical and extensive foreignbody reaction to commercially available (Ethicon, Inc.) black braidedsilk suture where no ingrowth or cell infiltration can be observed. FIG.18B demonstrates the engineered silk's ability to promote cell andtissue ingrowth. FIGS. 18A, 18B and 18C illustrate tissue response tosilk fiber constructs that are coated in wax (FIG. 18A), stripped ofsericin and coated with RGD (FIG. 18B), and stripped of sericin andseeded with progenitor adult stem cells (FIG. 18C).

FIGS. 19A-D shows H&E stained cross sectional images at 40× (top row,FIG. 19A & FIG. 19B) and 128× (bottom row, FIGS. 19C and 19D) of twoyarns (4×3×3 and 12×3), each containing the same number of fibers, butorganized differently with specific hierarchies following implantationin a rat model for 30 days. Results indication that yarn design andstructure can influence the extent of cell and tissue ingrowth as the12×3 yarn construct allowed for ingrowth, while it appears the 4×3×3thwarted it.

FIGS. 20 A, B and C are pictures of (A) single fiber wet laid non-wovenfabric extracted post fabric formation (fibers can first be extractedand formed into the non-woven—data not shown), (B) a knit fabricproduced from a form of chain stitching using 12-fiber yarn extractedpost fabric formation, and (C) a woven fabric produced frompre-extracted 12-fiber yarn with a 36-fiber pre-extracted yarn runningin the weft direction.

FIG. 21 is a schematic flow chart of the various methods and sequencesthat can be employed to create a biocompatible and biodegradable silkfibroin matrix. For example, extract single fiber, twist into yarns andknit into fabrics OR ply yarns, twist plied yarns, form fabric and thenextract. An almost infinite number of combination exists, but all willbe dependent on the hierarchy of the yarn, the number of fibers perlevel and the TPI per level as shown in Tables 4, 6, 7, and 8.

DETAILED DESCRIPTION

In methods described in greater detail, below, silk fibroin fibers arealigned in a parallel orientation; the fibers can remain in a strictlyparallel orientation, or they can be twisted or otherwise intertwined toform a yarn. The yarn can include any number of hierarchies, beginningat fiber level and expanding through bundle, strand, cord, etc., levels.Intertwining can be provided at each level. Furthermore, sericin isextracted from the silk fibers at any point in the hierarchy up to thepoint where the number of fibers exceeds that at which the extractingsolution can penetrate throughout the yarn. The maximum number ofsilkworm fibroin fibers (20/22 denier as purchased) that can be combinedand successfully extracted is about 50 (Table 4). These yarns can thenbe used as a fiber construct for, e.g., ligament or tissuereconstruction, or can be incorporated into a fabric for use, e.g., inthe generation of soft tissue mesh for repairs such as hernia repair,abdominal floor reconstruction and bladder slings. Formation of fiberconstructs will be discussed in the context of exemplary applications,below.

Although much of the discussion that follows is directed to asilk-fiber-based matrix (i.e. construct, scaffold) for producing ananterior cruciate ligament (ACL), a variety of other tissues, such asother ligaments and tendons, cartilage, muscle, bone, skin and bloodvessels, can be formed using a novel silk-fiber based matrix. In thecase of the ACL, a large yarn (540-3900 fibers per yarn, before plyingin parallel; see Table 8 & 11) with multiple hierarchical levels ofintertwining and relevant physiological properties was described. Inaddition to a silk-fiber-based ACL matrix, multiple smaller yarnconfigurations (1-50 silk fibers) (Table 1, 4 & 5) with relevantphysiological properties after combining either in parallel or into aspecific fabric formation, can serve as tissue matrices for guidedtissue formation (FIG. 2A-B). In addition to silk matrices for guidedtissue formation or engineering, this work is specifically directly toproducing a variety of silk-fiber based matrices tissue supportstructures for guided tissue repair (e.g., hernia repair, bladder slingsfor urinary stress incontinence) (FIG. 2A-B & FIG. 20A-C).

Constructs (i.e. fabrics or yarns) can be surface modified or seededwith the appropriate cells (FIG. 7A-D, FIG. 8A-B & FIG. 16A-C) andexposed to the appropriate mechanical stimulation, if necessary, forproliferating and differentiating into the desired ligament, tendon orother tissue in accordance with the above-described techniques.

Additionally, the present invention is not limited to using bone marrowstromal cells for seeding on the fiber construct, and other progenitor,pluripotent and stem cells, such as those in bone, muscle and skin forexample, may also be used to differentiate into ligaments and othertissues.

Fabrics can also be formed from similar constructs of purifiedfilaments, and used in various applications. Fabrics can be divided intovarious classes, including woven, non-woven, knitted fabrics, andstitch-bonded fabrics, each with numerous subtypes. Each of these typesmay be useful as an implant in particular circumstances. In discussingthese silk-based fabrics, we describe the natural silk, e.g., of Bombyxmori, as a “fibroin fiber.” The fibers should be at least one meterlong, and this length should be maintained throughout the process tofacilitate their handling during processing and incorporation into afabric. Given that a yarn may be defined as an assembly of fiberstwisted or otherwise held together in a continuous strand and that asingle fibroin fiber, as defined above, is comprised of multiple pliedbroins, sometimes from multiple cocoons, a single fibroin fiber may betermed a “yarn.” As well, fibroin fibers are twisted together orotherwise intertwined to form a “yarn.” Yarns are used to weave or knitfabrics for use in the invention. In an alternative procedure, silkyarns are disaggregated into shorter (5 mm to 100 mm) lengths or intosilk fibroin filaments. These filaments may then be (wet) laid to form anon-woven fabric (FIG. 20A).

When the yarns are formed into a fabric, the tension (force) exerted onthe yarns (typically, via machinery) is no greater than the yarn's yieldpoint (FIG. 3A-D). Accordingly, the yarns are handled at lower speedsand under smaller loads than are yarns that are typically used in, e.g.,textile manufacturing when forming the fabric so as to preserve theintegrity of the exposed fragile fibroin fibers. Likewise, contactpoints between handling machinery and the yarn are designed to avoidsharp angles and high-friction interactions so as to prevent lousing andfraying of fibers around the perimeter of the yarn (FIG. 4A-C).

Numerous applications of fabrics as implants are known in the medicaland surgical arts. One example is as a support in hernia repair. Forsuch repair, a fabric, most typically a warp-knit with a desired stitch(e.g., an atlas stitch designed to prevent unraveling of the mesh duringcutting), is sewn (or sometimes stapled or glued) or simply laid inplace without tensioning, onto the inside of the abdominal wall after itis repaired with conventional sutures. One function of the warp knitfabric is to provide short-term support for the repair. In a preferredembodiment of the present invention, the fibroin fibers within thefabric promote ingrowth of cells and subsequent tissue growth intofabric itself (FIGS. 13A & 13D) as well as through the fabric'sinterstices formed during knitting and into the region in need ofrepair. This embodiment aims to permanently strengthen the injured areathrough functional tissue ingrowth and remodeling as the silk matrixdegrades (FIGS. 13A,B & C).

Repair-strengthening fabrics are used in similar situations for repairor support of any part of the abdominal wall, particularly in herniarepair and abdominal floor reconstruction, or in repair or support ofother walls and septa in the body, for example of the chest, or oforgans such as the heart or the bladder, particularly after surgery ortumor removal. Implantable fabrics can also be used to support bladdersor other internal organs (included but not limited to the intestines,the ureters or urethra, and the uterus) to retain them in their normalpositions after surgery, damage or natural wear as a result of age orpregnancy, or to position them in an appropriate location. “Organ” hereincludes both “solid” organs, such as a liver, and tubular organs suchas an intestine or a ureter. Fabrics, especially bulky fabrics such assome non-woven types or those that can be created through 3-dimensionalknitting or braiding (FIG. 4A-C), can be used to fill cavities left bysurgery to provide a fiber construct onto which cells can migrate or towhich cells can be pre-attached (e.g. to improve the rate of repair).Usage sites include cavities in both soft tissues and hard tissues suchas bone. In other cases, fabrics are used to prevent adhesions, or toprevent the attachment and/or ingrowth of cells; this may be achievedthrough surface modification of the silk fibroin matrix or through theattachment of a drug or factor to the matrix.

The silk-fibroin-based fabrics of the invention can easily be modifiedin several ways to enhance healing or repair at the site. Thesemodifications may be used singly or in combination. Thesilk-fibroin-based fabrics of the invention can be surface modified tosupport cell attachment and spreading, cell and tissue ingrowth andremodeling, and device biodegradation through the use of RGD peptidecoupling or gas plasma irradiation (FIGS. 13A-E). The fabrics can bemodified to carry cell-attachment factors, such as the well-knownpeptide “RGD” (arginine-glycine-aspartic acid) or any of the manynatural and synthetic attachment-promoting materials, such as serum,serum factors and proteins including fibronectin, blood, marrow, groups,determinants, etc., known in the literature. Such materials can be inany of the usual biochemical classes of such materials, includingwithout limitation proteins, peptides, carbohydrates, polysaccharides,proteoglycans, nucleic acids, lipids, small (less than about 2000Daltons) organic molecules and combinations of these. Such plasmamodification can improve the fabric's surface functionality and/orcharge without affecting the materials bulk mechanical properties.Fabrics can be gas plasma irradiated after sericin extraction withoutcompromising the integrity of the sericin-extracted silk fibroin fibers(Table 9).

Additionally, the fabric can be treated so that it delivers a drug.Attachment of the drug to the fabric can be covalent, or covalent viadegradable bonds, or by any sort of binding (e.g., charge attraction) orabsorption. Any drug can be potentially used; non-limiting examples ofdrugs include antibiotics, growth factors such as bone morphogenicproteins (BMPs) or growth differentiation factors (GDFs), growthinhibitors, chemo-attractants, and nucleic acids for transformation,with or without encapsulating materials.

In another modification, cells can be added to the fabric before itsimplantation (FIG. 7A-D, FIG. 8A-B, and FIG. 9A-B). Cells can beseeded/absorbed on or into the fabric. Cells can also or in addition becultivated on the fabric, as a first step towards tissue replacement orenhancement. The cells may be of any type, but allogenous cells,preferably of the “immune protected,” immune privileged,” or stem celltypes are preferred, and autologous cells are particularly preferred.The cells are selected to be able to proliferate into required celltypes on or in the fiber construct (FIG. 9A-B).

Another class of modification is incorporation of other polymers (e.g.in fiber or gel form) into the fabric, to provide specific structuralproperties or to modify the native surfaces of the silk fibroin and itsbiological characteristics (see FIG. 16A-C: seeding of collagen fiberswith BMSCs). In one type of incorporation, fibers or yarns of silk andof another material are blended in the process of making the fabric. Inanother type, the silk-based fibers, yarns or fabrics are coated orover-wrapped with a solution or with fibers of another polymer. Blendingmay be performed (i) randomly, for example by plying (1 or multiplefibers of) both silk and the polymer together in parallel beforetwisting or (ii) in an organized fashion such as in braiding wherefibers or yarns being input into the larger yarn or fabric can alternatemachine feed positions creating a predicable outcome. Coating orwrapping may be performed by braiding or cabling over a central core,where the core can be the polymer, the silk fibroin or a composite ofboth, depending on the desired effect. Alternatively, one yarn can bewrapped in a controlled fashion over the other polymer, where thewrapping yarn can be used to stabilize the structure. Any biocompatiblepolymer is potentially usable. Examples of suitable polymers includeproteins, particularly structural proteins such as collagen and fibrin,and strength-providing degradable synthetic polymers, such as polymerscomprising anhydrides, hydroxy acids, and/or carbonates. Coatings may beprovided as gels, particularly degradable gels, formed of naturalpolymers or of degradable synthetic polymers. Gels comprising fibrin,collagen, and/or basement membrane proteins can be used. The gels can beused to deliver cells or nutrients, or to shield the surface from cellattachment. Further, proteins or peptides can be covalently attached tothe fibers or the fibers can be plasma modified in a charged gas (e.g.,nitrogen) to deposit amine groups; each of these coatings supports cellattachment and ingrowth, as silk is normally hydrophobic, and thesecoatings make the fibers more hydrophilic.

Non-limiting examples of some of these embodiments are described inexamples, below.

Wet laydown was selected for a prototype of fabric formation because itis the simplest procedure. The non-woven product (FIG. 20A) was createdfrom a single silk fibroin fiber prior to extraction at the fabriclevel. The product is correspondingly a relatively inexpensive material,and can be used in applications where its low tensile strength would besatisfactory. When more tensile strength is needed, a non-woven materialcould be bonded together, as is well known for fabrics and paper ormineralized for bone repair. Alternatively, silk yarn material producedby extraction of the sericin can be formed into a variety of morecomplex yarns, as described above. The size and design of the yarn canbe used to control porosity, independent of non-woven machinecapabilities. The yarns can also be knit (FIG. 20B) or woven (FIG. 20C)into a fabric. One type of fabric of interest is a simple mesh, similarto gauze, which can be used by itself (e.g. as a hemostat), or todeliver cells or drugs (e.g. a clotting factor) to a site, in asituation where flexibility is important.

When strength is important, a warp knit fabric (FIG. 20B), including thefamiliar tricots and jerseys, having an elasticity that can becontrolled through the helical design of the yarn used in the fabric,and typically substantial tensile strength, can be very useful forapplications (e.g., hernia repair, bladder slings, pelvic floorreconstructions, etc.) requiring provision of mechanical support for asignificant length of time, such as months.

In other applications, the material should have little elasticity andgreat strength. For such fabrics, a dense weave of thick yarns isappropriate, producing a material similar to standard woven fabrics(FIG. 20C). Such a material can optionally be supplemented by a coatingtreatment or a heat treatment to bond the crossovers of the yarnsegments, thereby preventing both raveling and stretching. Heattreatment must not entirely denature the silk protein. The fabric canoptionally be sewn, glued or stapled into place, as is currently donewith polypropylene mesh. The implant, like any of the other typesdiscussed, can be coated with various materials to enhance the localhealing and tissue ingrowth process, and/or with a coating to preventadhesion of the repair site to the viscera.

In another alternative, the fabric, mesh, non-woven, knit or otherrepair material can be made of unextracted silk, and then the finishedfabric can be extracted as described herein (FIG. 21) (for example, withalkaline soap solution at elevated temperature) to remove theimmunomodulatory sericins from the material. As a further alternative,the extraction of the sericin can take place at an intermediate stage,such as extraction of the formed yarn, bundle, or strand, in so far asthe number of fibers does not exceed that at which the extractingsolution can penetrate throughout the fibers (see FIG. 21 fornon-limiting options).

The above discussion has described making fabrics composed of yarns,where the most typical form of yarn in the fabric formations discussedabout would derive from twisting silkworm fibroin fibers together in anorganized manner and extracting sericin. Many yarn geometries andmethods of yarn formation may also be used as described (Tables 4, 5, 6,7 & 8). Such methods may include the formation of non-twisted bundles offibroin fibers, bound together by wrapping the bundles with silk oranother material as discussed above. Any of these yarns could, asdescribed above, be formed by blending silk fibers with other materials.Further still, the fibers can be intertwined, e.g., cabled, twisted,braided, meshed, knitted, etc. (see FIGS. 2A&B and 21). The term,“intertwined,” is used herein to indicate an organized (i.e.,non-random) repeating structure in terms of how the fibers contact andbind one another.

Blending could also be done at higher levels of organization, such asthe use of filaments of different materials to form a thicker yarn, orusing yarns of differing materials in weaving or knitting. In each case,the final material would include purified, essentially sericin-free silkas a significant component, used for one or all of its strength andbiocompatibility and (e.g., long-term) degradation characteristics (FIG.11A-B). The other polymer or polymers are selected for theirbiocompatibility, support (or inhibition through rapid tissue formationat desired locals) of cell attachment or infiltration (FIG. 16A-C),degradation profile in vivo, and mechanical properties. Biodegradablepolymers include any of the known biodegradable polymers, includingnatural products such as proteins, polysaccharides, glycosaminoglycans,and derivatized natural polymers, e.g., celluloses; and biodegradablesynthetic polymers and copolymers including polyhydroxy acids,polycarbonates, polyanhydrides, some polyamides, and copolymers andblends thereof. In particular, collagen and elastin are suitableproteins.

Silk-containing fabric constructs/matrices used for tissue repair may betreated so that they contain cells at the time of implantation (FIG.7A-D, FIG. 8A-B, FIG. 9A-B, & FIG. 18C) to improve tissue outcomes invivo. The cells may be xenogenic, more preferably allogenic, and mostpreferably autologous. Any type of cell is potentially of use, dependingon the location and the intended function of the implant. Pluripotentcells are preferred when the appropriate differentiation cues arepresent or provided in the environment. Other cell types includeosteogenic cells, fibroblasts, and cells of the tissue type of theimplantation site.

While silk from Bombyx mori and other conventional silkworms has beendescribed, any source of silk or silk-derived proteins can be used inthe invention, as long as it provokes no more than a mild foreign bodyreaction on implantation (i.e., is biocompatible) (see FIGS. 18B &C).These include without limitation silks from silkworms, spiders, andcultured cells, particularly genetically engineered cells, andtransgenic plants and animals. Silk produced by cloning may be from fullor partial sequences of native silk-line genes, or from synthetic genesencoding silk-like sequences.

While in many cases only a single fabric type will be used in formationof a medical device or prosthesis, it may be useful in some cases to usetwo or more types of fabric in a single device. For example, in herniarepair, it is desirable to have the tissue-facing side of the repairfabric attract cells, while the peritoneal face should repel cells, toprevent adhesions. This effect can be achieved by having one layer ofsilk that does not attract cells, and another layer that does (forexample, an untreated layer and an RGD-containing layer, as in theexample, below). Another example includes formation of a bladder sling.The basic sling should be conforming and somewhat elastic, and have along projected lifetime. However, the face of the sling closest to thebladder should have as little texture as feasible. This can beaccomplished by placing a layer of thin but tightly woven, non-woven orknitted fabric, fabricated from a yarn having a small diameter (e.g., asingle fiber), of the invention in the sling where it will contact thebladder. The non-woven fabric should be of as small a gauge (denier) asfeasible. Numerous other situations needing two or more types of fabricare possible.

Examples of the above-described structures were fabricated and evaluatedin a series of tests. In a first example, a fabric was formed frompurified silk fibrils. First, raw silk was processed into purifiedfibroin fibrils. Raw silkworm fibers were extracted in an aqueoussolution of 0.02 M Na₂CO₃ and 0.3% w/v IVORY soap solution for 60minutes at 90 degrees C. The extracted fibers were rinsed with water tocomplete the extraction of the glue-like sericin protein. The resultingsuspension of fibrils was wet-laid on a screen, needle-punched, anddried (FIG. 20A). The resulting fleecy material felt somewhat like woolto the touch, and was very porous. It was sufficiently interbonded byentanglement and needling that it was easily handled and cut to adesired shape.

In another example, the purified silk fibroin fibrils were treated withcell attracting agents (Table 9). First, yarns were made by twistingpurified fibers of silk fibroin together. Some yarns were made offilaments that were derivatized with the peptide RGD to attract cells,using procedures described in Sofia et al, J. Biomed. Mater. Res. 54:139-148, 2001. Sections of treated and untreated (black braided silksuture) yarns were implanted in the abdominal wall of rats (FIG. 18A-C).After 30 days of implantation, the black braided sutures containedcompact fibril bundles, with cell infiltration between fibril bundlesbut not within them. In contrast, the RGD treated fibril bundles wereextensively invaded by host cells, and were expanded and non-compact(FIG. 13A-E, 18B), but were not yet significantly degraded (FIG. 13A-E).

This example illustrates the use of derivatization to control the rateof degradation of implanted silk fibroin fibrils, as well asdemonstrating the ability of derivatized fibrils to recruit cells to afabric-like structure. Clearly, greater specificity of recruitment canbe obtained by using a more specific attractant. Similar techniques(chemical derivatization) or simpler methods such as absorption,adsorption, coating, and imbibement, can be used to provide othermaterials to the implantation site.

Each of the samples reported in the Tables below, was prepared inaccordance with the above description, wherein sericin was removed over60 minutes at a temperature of 90°+/−2° C. Using a temperature in thisrange for a sufficient period of time has been found to produce fibersfrom which sericin is substantially removed (FIG. 1A-C, Table 1, 2, 3)(to produce a fiber construct that is substantially free of sericin soas not to produce a significant immunological response and not tosignificantly impede the biodegradability of the fiber) whilesubstantially preserving the mechanical integrity of the fibroin (Table1). Note that when temperatures reach 94° C. (Table 1), UTS was notdramatically affected; however, stiffness significantly declinedindicating a silk thermo sensitive at temperatures of 94° C. and above.The fibers in each group were manually straightened (i.e., madeparallel) by pulling the ends of the fibers; alternatively,straightening could easily have been performed via an automated process.The force applied was marginally greater than what was required tostraighten the group.

The sample geometry designations in all Tables reflect the followingconstructs: # of fibers (tpi at fiber level in S direction)×# of groups(tpi at group level in Z direction)×# of bundles (tpi at bundle level inS direction)×# of strand (tpi at stand level in Z direction)×etc.,wherein the samples are twisted between levels unless otherwiseindicated. The twist-per-inch designation, such as 10s×9z tpi, reflects(the number of twists of the fibers/inch within the group)×(the numberof twists of the groups/inch within the bundle). In each sample, thepitch of the twist is substantially higher than is ordinarily found inconventional yarns that are twisted at a low pitch intended merely tohold the fibers together. Increasing the pitch of the twists (i.e.,increasing the twists per inch) decreases the tensile strength, but alsofurther decreases the stiffness and increases the elongation at break ofthe construct.

The ultimate tensile strength (UTS), percent elongation at break (%Elong), and stiffness were all measured using an INSTRON 8511servohydraulic material testing machine with FAST-TRACK software, whichstrained the sample at the high rate of ˜100% sample length per secondin a pull-to-failure analysis. In other words, up to the point offailure, the sample is stretched to double its length every second,which greatly restricts the capacity of the sample to relax and reboundbefore failure. However, FIG. 5A-B demonstrates the effect of strainrate can have on observed mechanical properties as well as wet or drytesting conditions which were shown (FIG. 6A-B) to have a dramaticeffect on silk matrix UTS and stiffness. Consistency is needed ifcomparisons are to be made between data sets. The resulting data wasanalyzed using Instron Series IX software. Ultimate tensile strength isthe peak stress of the resulting stress/strain curve, and stiffness isthe slope of the stress/strain plot up to the yield point. Unlessspecified, at least an N=5 was used for all tested groups to generateaverages and standard deviations. Standard statistical methods wereemployed to determine if statistically significant differences existedbetween groups, e.g., Student's t-test, one-way ANOVA.

The fibroin fibers in the samples in all of the above Tables and Figures(and throughout this disclosure) are native (i.e., the fibers are notdissolved and reformed); dissolution and reformulation of the fibersresults in a different fiber structure with different mechanicalproperties after reforming. Surprisingly, these samples demonstrate thatyarns of silk fibroin fibers, from which sericin has been completely ornearly completely removed, can possess high strengths and othermechanical properties that render the yarns suitable for variousbiomedical applications (Table 4, FIG. 2A-D & FIG. 20A-C), such as forforming a fiber construct or support for ligament replacement, herniarepair or pelvic floor reconstruction. Previously, it was believed thatfibroin needed to be dissolved and extruded into a reformulated fiber toprovide desired mechanical properties. Fatigue strength has generallybeen found to suffer in such reformed fibroin fibers. The methods of thepresent invention, allow for sericin removal without a significant lossof strength (Tables 1 & 4; FIGS. 3A-D & 4A-B).

In Table 8, samples 1 and 2 compare the properties of a 3-fiber group(sample 1) with those of a 4-fiber group (sample 2). Sample 2 had asquare configuration of fibers, while the fibers of sample 1 had atriangular configuration. As shown in the Table, the addition of theextra fiber in sample 2 lowered the per-fiber stiffness of the sampledemonstrating the ability to control yarn and fabric properties throughhierarchical design.

Table 4 illustrates the effects of different configurations ofcabled-fiber constructs and a twisted-fiber geometry. Note, inparticular, samples 7 and 8 include the same number of fibers and thesame number of geometrical levels. The twisted-fiber geometry of sample8 offers greater UTS and greater stiffness, while the cabled geometry ofsample 7 has lower strength and lower stiffness. Of samples 7-9, thecabled geometry of sample 7 has the highest strength-to-stiffness ratio;for use as an ACL fiber construct, a high strength-to-stiffness ratio isdesired (i.e., possessing a high strength and low stiffness).

Tables 1 and 4 demonstrate the effect of sericin extraction on thefibers. All samples were immersed in an extraction solution, asdescribed in Table 1. Samples 1-5 were immersed in a bath at roomtemperature, at 33° C. and 37° C. These temperatures are believed to betoo low to provide significant sericin extraction. Samples 6-9 wereextracted at 90° C., where complete sericin extraction is believed to beattainable, but for varying times. Similarly sample 10 was extracted atthe slightly higher temperature of 94° C. The data suggests that 30 to60 min at 90° C. is sufficient to significantly remove sericin (seeTables 2&3) and that 94° C. may be damaging the protein structure ofsilk as shown by a dramatic decrease in stiffness.

Finally, samples 11 to 16 have comparable cabled geometries; the fibersof samples 12, 14, and 16 were extracted, whereas the fibers of samples11, 13, and 15 were not. As can be seen in the Table, the extractionappears to have had little effect on (high) ultimate tensile strengthsper fiber.

The fibers of sample 10 of Table 4 were subject to a curl-shrinkingprocedure, wherein the fibers were twisted in one direction and then inthe opposite direction, rapidly; the fibers where then heated to lock inthe twist structure and tested non-extracted. The strength and stiffnessof the resulting yarn were comparatively lower than most of the othernon-extracted yarns tested. However, Tables 6&7 show the fibroinsremarkable ability, post extraction, to withstand up to 30 TPI. Table 6shows the ordering effect TRI has on silk matrices likely due to theordering of the multifilament structure following extraction.

FIG. 10 demonstrates the properties of a group of 30 parallel fibroinfibers seeded and non-seeded in culture conditions for 21 days. Thesethree samples exhibited very similar mechanical properties, therebyreflecting little if any degradation of silk matrices due to cell growththereon or due to time in vitro. Stiffness values are likely much lowerin this experiment in comparison with the other samples as a result ofthe 21 day wet incubation prior to mechanical testing (see Table 5).

Table 4, samples 14-16 are all braided samples. The fibers of sample 14were braided from eight carriers, with a spool mounted on each carrier,wherein two fibers were drawn from each spool. The fibers of sample 15were drawn from 16 carriers, with a spool mounted on each carrier;again, two fibers were drawn from each spool. Finally, sample 16 wasformed from 4 yarns, each yarn comprising 3 twisted groups of fourfibers (providing a total of 12 fibers per yarn); each of the yarns wasdrawn from a separate spool and carrier.

Table 9 demonstrates the effect of surface modification. Thedesignation, “PBS,” reflects that the samples were immersed in aphosphate-buffered saline solution for about 24 hours before testing.The effect of exposing the samples to the saline solution was measuredand provided an indication that the fiber construct can maintain itsmechanical properties and substantially preserve the inherent proteinstructure in a saline environment (e.g., inside a human body). The “RGD”designation reflects that the samples were immersed in an arg-gly-asp(RGD) saline solution for about 24 hours before testing. RGD can beapplied to the construct to attract cells to the construct and therebypromote cell growth thereon. Accordingly, any effect of RGD on themechanical properties of the construct is also of interest, though nosignificant degradation of the construct was apparent. Accordingly,these samples offer evidence that prolonged exposure to a salinesolution or gas ethylene oxide sterilization or to an RGD solutionresults in little, if any, degradation of the material properties of thefiber constructs. Though, the data associated with samples 28 and 29,wherein the geometrical hierarchy was extended to a higher level, revealthat the UTS/fiber drops as higher levels (and increased overall fibercount) are reached. This is an effect of hierarchical design (Table 8)rather than surface modification.

Table 4, samples 18 through 23 were tensioned under 6 pounds of constantforce for 1, 2, 3, 4, 5 and 6 days, respectively, before testing toevaluate the effect of tension on the mechanical properties over time.From the data, there does not appear to be much if any change in thematerial properties of the construct as the pretension procedure isextended over longer periods of time. Sample 25 was also “pre-tensioned”(after twisting) at 6 pounds force for a day before testing; forcomparison, sample 24, which had an identical geometrical configurationwas not pre-tensioned. Samples 24 and 25 accordingly reveal the effectof pre-tensioning the construct to remove the slack in the structure,which results in a slight reduction in both the construct's UTS and itselongation at break.

The silk-fiber-based construct serves as a matrix for infiltrating cellsor already infiltrated or seeded with cells, such as progenitor,ligament or tendon fibroblasts or muscle cells, which can proliferateand/or differentiate to form an anterior cruciate ligament (ACL) orother desired tissue type. The novel silk-fiber-based construct isdesigned having fibers in any of a variety of yarn geometries, such as acable, or in an intertwined structure, such as twisted yarn, braid,mesh-like yarn or knit-like yarn. The yarn exhibits mechanicalproperties that are identical or nearly identical to those of a naturaltissue, such as an anterior cruciate ligament (see Table 4, 1, infra);and simple variations in fiber construct organization and geometry canresult in the formation of any desired tissue type (see Table 10,infra). Alternatively, a plurality of yarns can be formed into a fabricor other construct that is implanted to position or support an organ.Additionally, the construct can be used to fill internal cavities aftersurgery or to prevent tissue adhesions or promote the attachment oringrowth of cells.

Pluripotent bone marrow stromal cells (BMSCs) that are isolated andcultured as described in the following example can be seeded on thesilk-fiber construct and cultured in a bioreactor under staticconditions. The cells seeded onto the fiber construct, if properlydirected, will undergo ligament and tendon specific differentiationforming viable and functional tissue. Moreover, the histomorphologicalproperties of a bioengineered tissue produced in vitro generated frompluripotent cells within a fiber construct are affected by the directapplication of mechanical force to the fiber construct during tissuegeneration. This discovery provides important new insights into therelationship between mechanical stress, biochemical and cellimmobilization methods and cell differentiation, and has applications inproducing a wide variety of ligaments, tendons and tissues in vitro frompluripotent cells.

A fiber construct comprising silk fibers having a cable geometry, isillustrated in FIGS. 2C and 2D. The fiber construct comprises ahierarchy in terms of the way that fibers are grouped in parallel andtwisted and how the resultant group is grouped and twisted, etc., acrossa plurality of levels in the hierarchy, as is further explained, below.The silk fibers are first tensioned in parallel using, for example, arack having spring-loaded clamps that serve as anchors for the fibers.The rack can be immersed in the sericin-extraction solution so that theclamps can maintain a constant tension on the fibers through extraction,rinsing and drying.

The extraction solution can be an alkaline soap solution or detergentand is maintained at about 90° C. The rack is immersed in the solutionfor a period of time (e.g., at least 0.5 to 1 hr, depending on solutionflow and mixing conditions) that is sufficient to remove all (+/−0.4%remaining, by weight) or substantially all sericin (allowing forpossible trace residue) from the fibers. Following extraction, the rackis removed from the solution and the fibers are rinsed and dried.Computer-controlled twisting machines, each of which mounts the fibersor constructs of fibers about a perimeter of a disc and rotates the discabout a central axis to twist the fibers (i.e. cabling) or constructs offibers twisted about each other according to standard processes used inthe textile industry, though at a higher pitch rate for the twists(e.g., between about 0 and about 11.8 twists per cm) than is typicallyproduced in traditional yarns. The cabling or twist rate, however,should not be so high as to cause plastic deformation of the fibers as aresult of the balloon tension created as the yarn is let-off from thefeed spool prior to twisting or cabling.

Extraction can be performed at any level of the construct provided thatthe solution can penetrate through the construct to remove the sericinfrom all fibers. It is believed that the upper limit for the number offibers in a compact arrangement that can still be fully permeated withthe solution is about 20-50 fibers. Though, of course, those fibers canbe arranged as one group of 20 parallel fibers or, for example, as 4groups of 5 parallel fibers, wherein the groups may be twisted, or evena construct comprising a still higher level such as 2 bundles of 2groups of 5 fibers, wherein the groups and bundles may be twisted.Increasing the number of hierarchical levels in the structure can alsoincrease the void space, thereby potentially increasing the maximumnumber of fibers from which sericin can be fully extracted from 20 to 50fibers.

Because the sericin, in some cases, is removed from the construct afterfibers are grouped or after a higher-level construct is formed, there isno need to apply wax or any other type of mechanically protectivecoating on the fibers or in order to also form a barrier to preventcontact with sericin on the fibers; and the construct can be free ofcoatings, altogether (particularly being free of coatings that are notfully degraded by the body or cause an inflammatory response).

As described in the examples below, mechanical properties of the silkfibroin (as illustrated in FIGS. 1A, 1B and 1C) were characterized, andgeometries for forming applicable matrices for ACL engineering werederived using a theoretical computational model (see FIG. 1D). Asix-cord construct was chosen for use as an ACL replacement to increasematrix surface area and to enhance support for tissue in-growth. Twoconstruct geometrical hierarchies for ACL repair comprise the following:

Matrix 1:1 ACL yarn=6 parallel cords; 1 cord=3 twisted strands (3twists/cm); 1 strand=6 twisted bundles (3 twists/cm); 1 bundle=30parallel washed fibers; and

Matrix 2: 1 ACL yarn=6 parallel cords; 1 cord=3 twisted strands (2twists/cm); 1 strand=3 twisted bundles (2.5 twists/cm); 1 bundle=3groups (3 twists/cm); 1 group=15 parallel extracted silk fibroin fibers.

The number of fibers and geometries for Matrix 1 and Matrix 2 wereselected such that the silk prostheses are similar to the ACLbiomechanical properties in ultimate tensile strength, linear stiffness,yield point and % elongation at break, serving as a solid starting pointfor the development of a tissue engineered ACL. The effects ofincreasing number of fibers, number of levels, and amount of twisting oneach of these biomechanical properties are shown in Table 8 and Tables6&7, respectively.

The ability to generate two matrices with differing geometries bothresulting in mechanical properties that mimic properties of the ACLindicates that a wide variety of geometrical configurations exist toachieve the desired mechanical properties. Alternative geometries forany desired ligament or tendon tissue may comprise any number,combination or organization of cords, strands, bundles, groups andfibers (see Table 10, infra) that result in a fiber construct withapplicable mechanical properties that mimic those of the ligament ortendon desired. For example, one (1) ACL prosthesis may have any numberof cords in parallel provided there is a mean for anchoring the finalfiber construct in vitro or in vivo. Further, various numbers oftwisting levels (where a single level is defined as a group, bundle,strand or cord) for a given geometry can be employed provided the fiberconstruct results in the desired mechanical properties. Furthermore,there is a large degree of freedom in designing the fiber constructgeometry and organization in engineering an ACL prosthesis; accordingly,the developed theoretical computational model can be used to predict thefiber construct design of a desired ligament or tendon tissue (see theexample, below). For example when multiple smaller matrix bundles aredesired (e.g., 36 fibers total) with only two levels of hierarchy topromote ingrowth, a TPI of 8-11 or ˜3-4 twists per cm is required andcan be predicted by the model without the need for empirical work.

Consequently, a variation in geometry (i.e., the number of cords used tomake a prosthesis or the number of fibers in a group) can be used togenerate matrices applicable to most ligaments and tendons. For example,for smaller ligaments or tendons of the hand, the geometry andorganization used to generate a single cord of Matrix 1 (or two cords orthree cords, etc.) may be appropriate given the fiber construct'sorganization results in mechanical properties suitable for theparticular physiological environment. Specifically, to accommodate asmaller ligament or tendon compared to Matrix 1 or Matrix 2, less fibersper level would be used to generate smaller bundles or strands. Multiplebundles could then be used in parallel. In the case of a larger ligamentsuch as the ACL, it might be desirable to have more smaller bundlestwisted at higher TPIs to reduce stiffness and promote ingrowth then tohave fewer larger bundles where ingrowth cannot occur thereby limiteddegradation of the matrix.

The invention is not, however, limited with respect to the cablegeometry as described, and any geometry or combination of geometries(e.g., parallel, twisted, braided, mesh-like) can be used that resultsin fiber construct mechanical properties similar to the ACL (i.e.,greater than 2000 N ultimate tensile strength, between 100-600 N/mmlinear stiffness for a native ACL or commonly used replacement graftsuch as the patellar tendon with length between 26-30 mm) or to thedesired ligament and tendon that is to be produced. The number of fibersand the geometry of both Matrix 1 and Matrix 2 were selected to generatemechanically appropriate ACL matrices, or other desired ligament ortendon matrices [e.g., posterior cruciate ligament (PCL)]. For example,a single cord of the six-cord Matrix 1 construct was used to reconstructthe medial collateral ligament (MC) in a rabbit (see FIG. 15A and FIG.15B). The mechanical properties of the silk six-cord constructs ofMatrix 1 and Matrix 2 are described in Table 10 and in FIGS. 3A-3D, asis further described in the example, infra. Additional geometries andtheir relating mechanical properties are listed in Table 11 as anexample of the large degree of design freedom that would result in afiber construct applicable in ACL tissue engineering in accordance withmethods described herein.

Advantageously, the silk-fiber based fiber construct can consist solelyof silk. Types and sources of silk include the following: silks fromsilkworms, such as Bombyx mori and related species; silks from spiders,such as Nephila clavipes; silks from genetically engineered bacteria,yeast mammalian cells, insect cells, and transgenic plants and animals;silks obtained from cultured cells from silkworms or spiders; nativesilks; cloned full or partial sequences of native silks; and silksobtained from synthetic genes encoding silk or silk-like sequences. Intheir raw form, the native silk fibroins obtained from the Bombyx morisilkworms are coated with a glue-like protein called sericin, which iscompletely or essentially completely extracted from the fibers beforethe fibers that make up the fiber construct are seeded with cells.

The fiber construct can comprise a composite of: (1) silk and collagenfibers; (2) silk and collagen foams, meshes, or sponges; (3) silkfibroin fibers and silk foams, meshes, or sponges; (4) silk andbiodegradable polymers [e.g., cellulose, cotton, gelatin, poly lactide,poly glycolic, poly(lactide-co-glycolide), poly caprolactone,polyamides, polyanhydrides, polyaminoacids, polyortho esters, polyacetals, proteins, degradable polyurethanes, polysaccharides,polycyanoacrylates, Glycosamino glycans (e.g., chondroitin sulfate,heparin, etc.), Polysaccharides (native, reprocessed or geneticallyengineered versions: e.g., hyaluronic acid, alginates, xanthans, pectin,chitosan, chitin, and the like), elastin (native, reprocessed orgenetically engineered and chemical versions), and collagens (native,reprocessed or genetically engineered versions], or (5) silk andnon-biodegradable polymers (e.g., polyamide, polyester, polystyrene,polypropylene, polyacrylate, polyvinyl, polycarbonate,polytetrafluorethylene, or nitrocellulose material. The compositegenerally enhances fiber construct properties such as porosity,degradability, and also enhances cell seeding, proliferation,differentiation or tissue development. FIGS. 16A, 16B and 16C illustratethe ability of collagen fibers to support BMSC growth and ligamentspecific differentiation.

The fiber construct can also be treated to enhance cell proliferationand/or tissue differentiation thereon. Exemplary fiber constructtreatments for enhancing cell proliferation and tissue differentiationinclude, but are not limited to, metals, irradiation, crosslinking,chemical surface modifications [e.g., RGD (arg-gly-asp) peptide coating,fibronectin coating, coupling growth factors], and physical surfacemodifications.

A second aspect of this disclosure relates to a mechanically andbiologically functional ACL formed from a novel silk-fiber-based fiberconstruct and autologous or allogenic (depending on the recipient of thetissue) bone marrow stromal cells (BMSCs) seeded on the fiber construct.The silk-fiber-based fiber construct induces stromal celldifferentiation towards ligament lineage without the need for anymechanical stimulation during bioreactor cultivation. BMSCs seeded onthe silk-fiber-based fiber construct and grown in a petri dish begin toattach and spread (see FIGS. 7A-D); the cells proliferate to cover thefiber construct (see FIGS. 8A-B, FIG. 9A and FIG. 9B) and differentiate,as shown by the expression of ligament specific markers (see FIG. 14).Markers for cartilage (collagen type II) and for bone (bonesialoprotein) were not expressed (see FIG. 14). Data illustrating theexpression of ligament specific markers is set forth in an example,below.

Another aspect of this disclosure relates to a method for producing anACL ex vivo. Cells capable of differentiating into ligament cells aregrown under conditions that simulate the movements and forcesexperienced by an ACL in vivo through the course of embryonicdevelopment into mature ligament function. Specifically, under sterileconditions, pluripotent cells are seeded within a three-dimensionalsilk-fiber-based fiber construct to which cells can adhere and which isadvantageously of cylindrical shape. The three-dimensionalsilk-fiber-based fiber construct used in the method serves as apreliminary fiber construct, which is supplemented and possibly evenreplaced by extracellular fiber construct components produced by thedifferentiating cells. Use of the novel silk-fiber-based fiber constructmay enhance or accelerate the development of the ACL. For instance, thenovel silk-fiber-based fiber construct can be designed to possessspecific mechanical properties (e.g., increased tensile strength) sothat it can withstand strong forces prior to reinforcement fromextracellular (e.g., collagen and tenascin) fiber construct components.Other advantageous properties of the novel silk-fiber based preliminaryfiber construct include, without limitation, biocompatibility andsusceptibility to biodegradation.

The pluripotent cells may be seeded within the preliminary fiberconstruct either pre- or post-fiber construct formation, depending uponthe particular fiber construct used and upon the method of fiberconstruct formation. Uniform seeding is usually preferable. In theory,the number of cells seeded does not limit the final ligament produced;however, optimal seeding may increase the rate of generation. Optimalseeding amounts will depend on the specific culture conditions. Thefiber construct can be seeded with from about 0.05 to 5 times thephysiological cell density of a native ligament.

One or more types of pluripotent cells are used in the method. Suchcells have the ability to differentiate into a wide variety of celltypes in response to the proper differentiation signals and to expressligament specific markers. More specifically, the method uses cells,such as bone marrow stromal cells, that have the ability todifferentiate into cells of ligament and tendon tissue. If the resultingbioengineered ligament is to be transplanted into a patient, the cellsshould be derived from a source that is compatible with the intendedrecipient. Although the recipient will generally be a human,applications in veterinary medicine also exist. The cells can beobtained from the recipient (autologous), although compatible donorcells may also be used to make allogenic ligaments. For example, whenmaking allogenic ligaments (e.g., using cells from another human such asbone marrow stromal cells isolated from donated bone marrow or ACLfibroblasts isolated from donated ACL tissue), human anterior cruciateligament fibroblast cells isolated from intact donor ACL tissue (e.g.,cadaveric or from total knee transplantations), ruptured ACL tissue(e.g., harvested at the time of surgery from a patient undergoing ACLreconstruction) or bone marrow stromal cells may be used. Thedetermination of compatibility is within the means of the skilledpractitioner.

Ligaments or tendons including, but not limited to, the posteriorcruciate ligament, rotator cuff tendons, medial collateral ligament ofthe elbow and knee, flexor tendons of the hand, lateral ligaments of theankle and tendons and ligaments of the jaw or temporomandibular jointother than ACL, cartilage, bone and other tissues may be engineered withthe fiber construct in accordance with methods of this disclosure. Inthis manner, the cells to be seeded on the fiber construct are selectedin accordance with the tissue to be produced (e.g., pluripotent or ofthe desired tissue type). Cells seeded on the fiber construct, asdescribed herein, can be autologous or allogenic. The use of autologouscells effectively creates an allograft or autograft for implantation ina recipient.

As recited, to form an ACL, cells, such as bone marrow stromal cells,are seeded on the fiber construct. Bone marrow stromal cells are a typeof pluripotent cell and are also referred to in the art as mesenchymalstem cells or simply as stromal cells. As recited, the source of thesecells can be autologous or allogenic. Additionally, adult or embryonicstem or pluripotent cells can be used if the proper environment (eitherin vivo or in vitro), seeded on the silk-fiber based fiber construct,can recapitulate an ACL or any other desired ligament or tissue inextracellular fiber construct composition (e.g., protein, glycoproteincontent), organization, structure or function.

Fibroblast cells can also be seeded on the inventive fiber construct.Since fibroblast cells are often not referred to as pluripotent cells,fibroblasts are intended to include mature human ACL fibroblasts(autologous or allogenic) isolated from ACL tissue, fibroblasts fromother ligament tissue, fibroblasts from tendon tissue, from neonatalforeskin, from umbilical cord blood, or from any cell, whether mature orpluripotent, mature dedifferentiated, or genetically engineered, suchthat when cultured in the proper environment (either in vivo or invitro), and seeded on the silk-fiber based fiber construct, canrecapitulate an ACL or any other desired ligament or tissue inextracellular fiber construct composition (e.g., protein, glycoproteincontent), organization, structure or function.

The faces of the fiber construct cylinder are each attached to anchors,through which a range of forces is to be applied to the fiber construct.To facilitate force delivery to the fiber construct, the entire surfaceof each respective face of the fiber construct can contact the face ofthe respective anchors. Anchors with a shape that reflects the site ofattachment (e.g., cylindrical) are best suited for use in this method.Once assembled, the cells in the anchored fiber construct are culturedunder conditions appropriate for cell growth and regeneration. The fiberconstruct is subjected to one or more mechanical forces applied throughthe attached anchors (e.g., via movement of one or both of the attachedanchors) during the course of culture. The mechanical forces are appliedover the period of culture to mimic conditions experienced by the nativeACL or other tissues in vivo.

The anchors must be made of a material suitable for fiber constructattachment, and the resulting attachment should be strong enough toendure the stress of the mechanical forces applied. In addition, theanchors can be of a material that is suitable for the attachment ofextracellular fiber construct that is produced by the differentiatingcells. The anchors support bony tissue in-growth (either in vitro or invivo) while anchoring the developing ligament. Some examples of suitableanchor material include, without limitation, hydroxyapatite, Gonioporacoral, demineralized bone, bone (allogenic or autologous). Anchormaterials may also include titanium, stainless steel, high densitypolyethylene, DACRON and TEFLON.

Alternatively, anchor material may be created or further enhanced byinfusing a selected material with a factor that promotes either ligamentfiber construct binding or bone fiber construct binding or both. Theterm infuse is considered to include any method of application thatappropriately distributes the factor onto the anchor (e.g., coating,permeating, contacting). Examples of such factors include withoutlimitation, laminin, fibronectin, any extracellular fiber constructprotein that promotes adhesion, silk, factors that containarginine-glycine-aspartate (RGD) peptide binding regions or the RGDpeptides themselves. Growth factors or bone morphogenic protein can alsobe used to enhance anchor attachment. In addition, anchors may bepre-seeded with cells (e.g., stem cells, ligament cells, osteoblasts,osteogenic progenitor cells) that adhere to the anchors and bind thefiber construct, to produce enhanced fiber construct attachment both invitro and in vivo.

An exemplary anchor system is disclosed in applicant's co-pendingapplication U.S. Ser. No. 09/950,561, which is incorporated herein byreference in its entirety. The fiber construct is attached to theanchors via contact with the anchor face or alternatively by actualpenetration of the fiber construct material through the anchor material.Because the force applied to the fiber construct via the anchorsdictates the final ligament produced, the size of the final ligamentproduced is, in part, dictated by the size of the attachment site of theanchor. An anchor of appropriate size to the desired final ligamentshould be used. An example of an anchor shape for the formation of anACL is a cylinder. However, other anchor shapes and sizes will alsofunction adequately. For example, anchors can have a size andcomposition appropriate for direct insertion into bone tunnels in thefemur and tibia of a recipient of the bioengineered ligament.

Alternatively, anchors can be used only temporarily during in vitroculture, and then removed when the fiber construct alone is implanted invivo.

Further still, the novel silk-fiber-based fiber construct can be seededwith BMSCs and cultured in a bioreactor. Two types of growthenvironments currently exist that may be used in accordance with methodsof this disclosure: (1) the in vitro bioreactor apparatus system, and(2) the in vivo knee joint, which serves as a “bioreactor” as itprovides the physiologic environment including progenitor cells andstimuli (both chemical and physical) necessary for the development of aviable ACL given a fiber construct with proper biocompatible andmechanical properties. The bioreactor apparatus provides optimal cultureconditions for the formation of a ligament in terms of differentiationand extracellular fiber construct (ECM) production, and which thusprovides the ligament with optimal mechanical and biological propertiesprior to implantation in a recipient. Additionally, when the silk-fiberbased fiber construct is seeded and cultured with cells in vitro, apetri dish may be considered to be the bioreactor within whichconditions appropriate for cell growth and regeneration exist, i.e., astatic environment.

Cells can also be cultured on the fiber construct fiber constructwithout the application of any mechanical forces, i.e., in a staticenvironment. For example, the silk-fiber based fiber construct alone,with no in vitro applied mechanical forces or stimulation, when seededand cultured with BMSCs, induces the cells to proliferate and expressligament and tendon specific markers (see the examples, describedherein). The knee joint may serve as a physiological growth anddevelopment environment that can provide the cells and the correctenvironmental signals (chemical and physical) to the fiber constructfiber construct such that an ACL technically develops. Therefore, theknee joint (as its own form of bioreactor) plus the fiber construct(either non-seeded, seeded and not differentiated in vitro, or seededand differentiated in vitro prior to implantation) will result in thedevelopment of an ACL, or other desired tissue depending upon the celltype seeded on the fiber construct and the anatomical location of fiberconstruct implantation. FIG. 15 A-B illustrates the effects of themedial collateral knee joint environment on medial collateral ligament(MCL) development when only a non-seeded silk-based fiber construct withappropriate MCL mechanical properties is implanted for 6 weeks in vivo.Whether the cells are cultured in a static environment with nomechanical stimulation applied, or in a dynamic environment, such as ina bioreactor apparatus, conditions appropriate for cell growth andregeneration are advantageously present for the engineering of thedesired ligament or tissue.

In experiments described in the examples, below, the applied mechanicalstimulation was shown to influence the morphology, and cellularorganization of the progenitor cells within the resulting tissue. Theextracellular fiber construct components secreted by the cells and theorganization of the extracellular fiber construct throughout the tissuewas also significantly influenced by the forces applied to the fiberconstruct during tissue generation. During in vitro tissue generation,the cells and extracellular fiber construct aligned along the axis ofload, reflecting the in vivo organization of a native ACL that is alsoalong the various load axes produced from natural knee joint movementand function. These results suggest that the physical stimuliexperienced in nature by cells of developing tissue, such as the ACL,play a significant role in progenitor cell differentiation and tissueformation. They further indicate that this role can be effectivelyduplicated in vitro by mechanical manipulation to produce a similartissue. The more closely the forces produced by mechanical manipulationresemble the forces experienced by an ACL in vivo, the more closely theresultant tissue will resemble a native ACL.

When mechanical stimulation is applied in vitro to the fiber constructvia a bioreactor, there exists independent but concurrent control overboth cyclic and rotation strains as applied to one anchor with respectto the other anchor. Alternatively, the fiber construct alone may beimplanted in vivo, seeded with ACL cells from the patient and exposed invivo to mechanical signaling via the patient.

When the fiber construct is seeded with cells prior to implantation, thecells are cultured within the fiber construct under conditionsappropriate for cell growth and differentiation. During the cultureprocess, the fiber construct may be subjected to one or more mechanicalforces via movement of one or both of the attached anchors. Themechanical forces of tension, compression, torsion and shear, andcombinations thereof, are applied in the appropriate combinations,magnitudes, and frequencies to mimic the mechanical stimuli experiencedby an ACL in vivo.

Various factors will influence the amount of force that can be toleratedby the fiber construct (e.g., fiber construct composition, celldensity). Fiber construct strength is expected to change through thecourse of tissue development. Therefore, applied mechanical forces orstrains will increase, decrease or remain constant in magnitude,duration, frequency and variety over the period of ligament generation,to appropriately correspond to fiber construct strength at the time ofapplication.

When producing an ACL, the more accurate the intensity and combinationof stimuli applied to the fiber construct during tissue development, themore the resulting ligament will resemble a native ACL. Two issues mustbe considered regarding the natural function of the ACL when devisingthe in vitro mechanical force regimen that closely mimics the in vivoenvironment: (1) the different types of motion experienced by the ACLand the responses of the ACL to knee joint movements and (2) the extentof the mechanical stresses experienced by the ligament. Specificcombinations of mechanical stimuli are generated from the naturalmotions of the knee structure and transmitted to the native ACL.

To briefly describe the motions of the knee, the connection of the tibiaand femur by the ACL provides six degrees of freedom when consideringthe motions of the two bones relative to each other. The tibia can movein three directions and can rotate relative to the axes for each ofthese three directions. The knee is restricted from achieving the fullranges of these six degrees of freedom due to the presence of ligamentsand capsular fibers and the knee surfaces themselves (Biden et al.,“Experimental Methods Used to Evaluate Knee Ligament Function,” KneeLigaments Structure, Function, Injury and Repair, Ed. D. Daniel et al.,Raven Press, pp. 135-151, 1990). Small translational movements are alsopossible. The attachment sites of the ACL are responsible for itsstabilizing roles in the knee joint. The ACL functions as a primarystabilizer of anterior-tibial translation, and as a secondary stabilizerof valgus-varus angulation, and tibial rotation (Shoemaker et al., “TheLimits of Knee Motion,” Knee Ligaments: Structure, Function, Injury andRepair, Ed. D. Daniel et al., Raven Press, pp. 1534-161, 1990).Therefore, the ACL is responsible for stabilizing the knee in three ofthe six possible degrees of freedom. As a result, the ACL has developeda specific fiber organization and overall structure to perform thesestabilizing functions. These conditions are simulated in vitro toproduce a tissue with similar structure and fiber organization.

The extent of mechanical stresses experienced by the ACL can besimilarly summarized. The ACL undergoes cyclic loads of about 400 Nbetween one and two million cycles per year (Chen et al., J. Biomed.Mat. Res. 14: 567-586, 1980). Also considered are linear stiffness (˜182N/mm), ultimate deformation (100% of ACL) and energy absorbed at failure(12.8 N-m) (Woo et al., The tensile properties of human anteriorcruciate ligament (ACL) and ACL graft tissues, Knee Ligaments:Structure, Function, Injury and Repair, Ed. D. Daniel et al. RavenPress, pp. 279-289, 1990) when developing an ACL surgical replacement.

The examples section, below, details the production of a prototypebioengineered anterior cruciate ligament (ACL) ex vivo. Mechanicalforces mimicking a subset of the mechanical stimuli experienced by anative ACL in vivo (rotational deformation and linear deformation) wereapplied in combination, and the resulting ligament that was formed wasstudied to determine the effects of the applied forces on tissuedevelopment. Exposure of the developing ligament to physiologicalloading during in vitro formation induced the cells to adopt a definedorientation along the axes of load, and to generate extracellularmatrices along the axes as well. These results indicate that theincorporation of complex multi-dimensional mechanical forces into theregime to produce a more complex network of load axes that mimics theenvironment of the native ACL will produce a bioengineered ligament thatmore closely resembles a native ACL.

The different mechanical forces that may be applied include, withoutlimitation, tension, compression, torsion, and shear. These forces areapplied in combinations that simulate forces experienced by an ACL inthe course of natural knee joint movements and function. These movementsinclude, without limitation, knee joint extension and flexion as definedin the coronal and sagittal planes, and knee joint flexion. Optimally,the combination of forces applied mimics the mechanical stimuliexperienced by an anterior cruciate ligament in vivo as accurately as isexperimentally possible. Varying the specific regimen of forceapplication through the course of ligament generation is expected toinfluence the rate and outcome of tissue development, with optimalconditions to be determined empirically. Potential variables in theregimen include, without limitation: (1) strain rate, (2) percentstrain, (3) type of strain (e.g., translation and rotation), (4)frequency, (5) number of cycles within a given regime, (6) number ofdifferent regimes, (7) duration at extreme points of ligamentdeformation, (8) force levels, and (9) different force combinations. Awide variety of variations exist. The regimen of mechanical forcesapplied can produce helically organized fibers similar to those of thenative ligament, described below.

The fiber bundles of a native ligament are arranged into a helicalorganization. The mode of attachment and the need for the knee joint torotate ˜140° of flexion has resulted in the native ACL inheriting a 90°twist and with the peripheral fiber bundles developing a helicalorganization. This unique biomechanical feature allows the ACL tosustain extremely high loading. In the functional ACL, this helicalorganization of fibers allows anterior-posterior and posterior-anteriorfibers to remain relatively isometric in respect to one another for alldegrees of flexion, thus load can be equally distributed to all fiberbundles at any degree of knee joint flexion, stabilizing the kneethroughout all ranges of joint motion. Mechanical forces that simulate acombination of knee joint flexion and knee joint extension can beapplied to the developing ligament to produce an engineered ACL thatpossesses this same helical organization. The mechanical apparatus usedin the experiments presented in the examples, below, provides controlover strain and strain rates (both translational and rotational). Themechanical apparatus will monitor the actual load experienced by thegrowing ligaments, serving to ‘teach’ the ligaments over time throughmonitoring and increasing the loading regimes.

Another aspect of this disclosure relates to the bioengineered anteriorcruciate ligament produced by the above-described methods. Thebioengineered ligament produced by these methods is characterized bycellular orientation and/or a fiber construct crimp pattern in thedirection of the mechanical forces applied during generation. Theligament is also characterized by the production/presence ofextracellular fiber construct components (e.g., collagen type I and typeIII, fibronectin, and tenascin-C proteins) along the axis of mechanicalload experienced during culture. The ligament fiber bundles can bearranged into a helical organization, as discussed above.

The above methods using the novel silk-fiber-based fiber construct arenot limited to the production of an ACL, but can also be used to produceother ligaments and tendons found in the knee (e.g., posterior cruciateligament) or other parts of the body (e.g., hand, wrist, ankle, elbow,jaw and shoulder), such as for example, but not limited to posteriorcruciate ligament, rotator cuff tendons, medial collateral ligament ofthe elbow and knee, flexor tendons of the hand, lateral ligaments of theankle and tendons and ligaments of the jaw or temporomandibular joint.All moveable joints in a human body have specialized ligaments thatconnect the articular extremities of the bones in the joint. Eachligament in the body has a specific structure and organization that isdictated by its function and environment. The various ligaments of thebody, their locations and functions are listed in Anatomy, Descriptiveand Surgical (Gray, H., Eds. Pick, T. P., Howden, R., Bounty Books, NewYork, 1977), the pertinent contents of which are incorporated herein byreference. By determining the physical stimuli experienced by a givenligament or tendon, and incorporating forces which mimic these stimuli,the above-described method for producing an ACL ex vivo can be adaptedto produce bioengineered ligaments and tendons ex vivo that simulatesany ligament or tendon in the body.

The specific type of ligament or tendon to be produced is predeterminedprior to tissue generation since several aspects of the method vary withthe specific conditions experienced in vivo by the native ligament ortendon. The mechanical forces to which the developing ligament or tendonis subjected during cell culture are determined for the particularligament or tendon type being cultivated. The specific conditions can bedetermined by studying the native ligament or tendon and its environmentand function. One or more mechanical forces experienced by the ligamentor tendon in vivo are applied to the fiber construct during culture ofthe cells in the fiber construct. The skilled practitioner willrecognize that a ligament or tendon that is superior to those currentlyavailable can be produced by the application of a subset of forcesexperienced by the native ligament or tendon. However, optimally, thefull range of in vivo forces will be applied to the fiber construct inthe appropriate magnitudes and combinations to produce a final productthat most closely resembles the native ligament or tendon. These forcesinclude, without limitation, the forces described above for theproduction of an ACL. Because the mechanical forces applied vary withligament or tendon type, and the final size of the ligament or tendonwill be influenced by the anchors used, optimal anchor composition, sizeand fiber construct attachment sites are to be determined for each typeof ligament or tendon by the skilled practitioner. The type of cellsseeded on the fiber construct is obviously determined based on the typeof ligament or tendon to be produced.

Other tissue types can be produced ex vivo using methods similar tothose described above for the generation of ligaments or tendons exvivo. The above-described methods can also be applied to produce a rangeof engineered tissue products that involve mechanical deformation as amajor part of their function, such as muscle (e.g., smooth muscle,skeletal muscle, cardiac muscle), bone, cartilage, vertebral discs, andsome types of blood vessels. Bone marrow stromal cells possess theability to differentiate into these as well as other tissues. Thegeometry of the silk-based fiber construct or composite fiber constructcan easily be adapted to the correct anatomical geometricalconfiguration of the desired tissue type. For example, silk fibroinfibers can be reformed in a cylindrical tube to recreate arteries.

The results presented in the examples, below, indicate that growth in anenvironment that mimics the specific mechanical environment of a giventissue type will induce the appropriate cell differentiation to producea bioengineered tissue that significantly resembles native tissue. Theranges and types of mechanical deformation of the fiber construct can beextended to produce a wide range of tissue structural organization. Thecell culture environment can reflect the in vivo environment experiencedby the native tissue and the cells it contains, throughout the course ofembryonic development to mature function of the cells within the nativetissue, as accurately as possible. Factors to consider when designingspecific culture conditions to produce a given tissue include, withoutlimitation, the fiber construct composition, the method of cellimmobilization, the anchoring method of the fiber construct or tissue,the specific forces applied, and the cell culture medium. The specificregimen of mechanical stimulation depends upon the tissue type to beproduced, and is established by varying the application of mechanicalforces (e.g., tension only, torsion only, combination of tension andtorsion, with and without shear, etc.), the force amplitude (e.g., angleor elongation), the frequency and duration of the application, and theduration of the periods of stimulation and rest.

The method for producing the specific tissue type ex vivo is anadaptation of the above-described method for producing an ACL.Components involved include pluripotent cells, a three-dimensional fiberconstruct to which cells can adhere, and a plurality of anchors thathave a face suitable for fiber construct attachment. The pluripotentcells (such as bone marrow stromal cells) are seeded in the threedimensional fiber construct by means to uniformly immobilize the cellswithin the fiber construct. The number of cells seeded is also notviewed as limiting, however, seeding the fiber construct with a highdensity of cells may accelerate tissue generation.

The specific forces applied are to be determined for each tissue typeproduced through examination of native tissue and the mechanical stimuliexperienced in vivo. A given tissue type experiences characteristicforces that are dictated by location and function of the tissue withinthe body. For instance, cartilage is known to experience a combinationof shear and compression/tension in vivo; bone experiences compression.

Additional stimuli (e.g., chemical stimuli, electro-magnetic stimuli)can also be incorporated into the above-described methods for producingbioengineered ligaments, tendons and other tissues. Cell differentiationis known to be influenced by chemical stimuli from the environment,often produced by surrounding cells, such as secreted growth ordifferentiation factors, cell-cell contact, chemical gradients, andspecific pH levels, to name a few. Other more unique stimuli areexperienced by more specialized types of tissues (e.g., the electricalstimulation of cardiac muscle). The application of such tissue specificstimuli (e.g., 1-10 ng/ml transforming growth factor beta-1 (TGF-β1)independently or in concert with the appropriate mechanical forces isexpected to facilitate differentiation of the cells into a tissue thatmore closely approximates the specific natural tissue.

Tissues produced by the above-described methods provide an unlimitedpool of tissue equivalents for surgical implantation into a compatiblerecipient, particularly for replacement or repair of damaged tissue.Engineered tissues may also be utilized for in vitro studies of normalor pathological tissue function, e.g., for in vitro testing of cell- andtissue-level responses to molecular, mechanical, or geneticmanipulations. For example, tissues based on normal or transfected cellscan be used to assess tissue responses to biochemical or mechanicalstimuli, identify the functions of specific genes or gene products thatcan be either over-expressed or knocked-out, or to study the effects ofpharmacological agents. Such studies will likely provide more insightinto ligament, tendon and tissue development, normal and pathologicalfunction, and eventually lead toward fully functional tissue engineeredreplacements, based in part on already established tissue engineeringapproaches, new insights into cell differentiation and tissuedevelopment, and the use of mechanical regulatory signals in conjunctionwith cell-derived and exogenous biochemical factors to improvestructural and functional tissue properties.

The production of engineered tissues, such as ligaments and tendons,also has the potential for applications such as harvesting bone marrowstromal cells from individuals at high risk for tissue injury (e.g., ACLrupture) prior to injury. These cells could be either stored untilneeded or seeded into the appropriate fiber construct and cultured anddifferentiated in vitro under mechanical stimuli to produce a variety ofbioengineered prosthetic tissues to be held in reserve until needed bythe donor. The use of bioengineered living tissue prosthetics thatbetter match the biological environment in vivo and that provide therequired physiological loading to sustain, for example, the dynamicequilibrium of a normal, fully functional ligament should reducerehabilitation time for a recipient of a prosthesis from months toweeks, particularly if the tissue is pre-grown and stored. Benefitsinclude a more rapid regain of functional activity, shorter hospitalstays, and fewer problems with tissue rejections and failures.

Additional aspects of this invention are further exemplified in thefollowing examples. It will be apparent to those skilled in the art thatmany modifications, both to the materials and methods, may be practicedwithout departing from the invention.

In a first example, raw Bombyx mori silkworm fibers, shown in FIG. 1A,were extracted to remove sericin, the glue-like protein coating thenative silk fibroin (see FIGS. 1A-C). The appropriate number of fibersper group were arranged in parallel and extracted in an aqueous solutionof 0.02 M Na₂CO₃ and 0.3% (w/v) IVORY soap solution for 60 minutes at90° C., then rinsed thoroughly with water to extract the glue-likesericin proteins.

Costello's equation for a three-strand, helical rope geometry wasderived to predict mechanical properties of the silk-fiber-basedconstruct. The derived model is a series of equations that whencombined, take into account extracted silk fiber material properties anddesired fiber construct geometrical hierarchy to compute the overallstrength and stiffness of the fiber construct as a function of pitchangle for a given level of geometrical hierarchy.

The material properties of a single silk fiber include fiber diameter,modulus of elasticity, Poisson's ratio, and the ultimate tensilestrength (UTS). Geometrical hierarchy may be defined as the number oftwisting levels in a given fiber construct level. Each level (e.g.,group, bundle, strand, cord, ligament) is further defined by the numberof groups of fibers twisted about each other and the number of fibers ineach group of the first level twisted where the first level is define asa group, the second level as a bundle, the third as a strand and thefourth as a cord, the fifth as the ligament.

The model assumes that each group of multiple fibers act as a singlefiber with an effective radius determined by the number of individualfibers and their inherent radius, i.e., the model discounts frictionbetween the individual fibers due to its limited role in given arelatively high pitch angle.

Two applicable geometries (Matrix 1 and Matrix 2) of the many fiberconstruct geometrical configurations (see Table 10, supra) computed toyield mechanical properties mimicking those of a native ACL were derivedfor more detailed analysis. A six-cord construct was selected for use asthe ACL replacement. Matrix configurations are as follows: Matrix 1:1ACL prosthesis=6 parallel cords; 1 cord=3 twisted strands (3 twists/cm);1 strand=6 twisted bundles (3 twists/cm); 1 bundle=30 parallel washedfibers; and Matrix 2: 1 ACL matrix=6 parallel cords; 1 cord=3 twistedstrands (2 twists/cm); 1 strand=3 twisted bundles (2.5 twists/cm); 1bundle=3 groups (3 twists/cm); 1 group=15 parallel extracted silkfibroin fibers. The number of fibers and geometries were selected suchthat the silk prostheses are similar to the ACL biomechanical propertiesin UTS, linear stiffness, yield point and % elongation at break (seeTable 10, supra), thus serving as a solid starting point for thedevelopment of a tissue engineered ACL.

Mechanical properties of the silk fibroin were characterized using aservohydraulic Instron 8511 tension/compression system with Fast-Tracksoftware (Instron Corp., Canton, Mass., USA) (see FIG. 1D). Singlepull-to-failure and fatigue analyses were performed on single silkfibers, extracted fibroin and organized cords. Fibers and fibroin wereorganized in both the parallel helical geometries of Matrix 1 (see FIG.2C) and of Matrix 2 (see FIG. 2D) for characterization. Single pull tofailure testing was performed at a strain rate of 100%/sec; forceelongation histograms were generated and data analyzed using InstronSeries IX software. Both Matrix 1 and Matrix 2 yielded similarmechanical and fatigue properties to the ACL in UTS, linear stiffness,yield point and percent elongation at break (see Table 10 and FIGS.3A-D).

Fatigue analyses were performed using a servohydraulic Instron 8511tension/compression system with Wavemaker software on single cords ofboth Matrix 1 and Matrix 2. Data was extrapolated to represent the6-cord ACL prostheses, which is shown in FIGS. 3B and 3D. Cord ends wereembedded in an epoxy mold to generate a 3-cm-long construct betweenanchors. Cycles to failure at UTS's of 1,680 N and 1,200 N (n=5 for eachload) for Matrix 1 (see FIG. 3B) and at UTS's of 2280 N, 2100 N and 1800N loads (n=3 for each load) for Matrix 2 (see FIG. 3D) were determinedusing a H-sine wave function at 1 Hz generated by Wavemaker 32 version6.6 software (Instron Corp.). Fatigue testing was conducted in a neutralphosphate buffered saline (PBS) solution at room temperature.

Complete sericin removal was observed after 60 min at 90° C. asdetermined by SEM (see FIGS. 1A-C). Removal of sericin from silk fibersaltered the ultrastructure of the fibers, resulting in a smoother fibersurface, and the underlying silk fibroin was revealed (shown in FIGS.1A-C), with average diameter ranging between 20-40 μm. The fibroinexhibited a significant 15.2% decrease in ultimate tensile strength(1.033+/−0.042 N/fiber to 0.876+/−0.1 N/fiber) (p<0.05, paired Studentst-test) (see FIG. 1D). The mechanical properties of the optimized silkmatrices (see FIG. 2A-D & FIG. 3A-D) are summarized in Table 11 aboveand in FIG. 3A (for Matrix 1) and in FIG. 3C (for Matrix 2). It isevident from these results that the optimized silk matrices exhibitedvalues comparable to those of native ACL, which have been reported tohave an average ultimate tensile strength (UTS) of ˜2100 N, stiffness of˜250 N/nm, yield point ˜2100 N and 33% elongation at break (See Woo,SL-Y, et al., The Tensile Properties of Human Anterior Cruciate Ligament(ACL) and ACL Graft Tissue in Knee Ligaments: Structure, Function,Injury and Repair, 279-289, Ed. D. Daniel et al., Raven Press 1990).

Regression analysis of fiber construct fatigue data, shown in FIG. 3Bfor Matrix 1 and in FIG. 3D for Matrix 2, when extrapolated tophysiological load levels (400 N) predict the number of cycles tofailure in vivo, indicate a fiber construct life of 3.3 million cyclesfor Matrix 1 and a life of greater than 10 million cycles for Matrix 2.The helical fiber construct design utilizing washed silk fibers resultedin a fiber construct with physiologically equivalent structuralproperties, confirming its suitability as a scaffold for ligament tissueengineering.

In another example involving cell isolation and culture, bone marrowstromal cells (BMSC), pluripotent cells capable of differentiating intoosteogenic, chondrogenic, tendonogenic, adipogenic and myogeniclineages, were chosen since the formation of the appropriate conditionscan direct their differentiation into the desired ligament fibroblastcell line (Markolf et al., J. Bone Joint Surg. 71A: 887-893, 1989;Caplan et al., Mesenchymal stem cells and tissue repair, The AnteriorCruciate Ligament: Current and Future Concepts, Ed. D. W. Jackson etal., Raven Press, Ltd, New York, 1993; Young et al., J. Orthopaedic Res.16: 406-413, 1998).

Human BMSCs were isolated from bone marrow from the iliac crest ofconsenting donors at least 25 years of age by a commercial vendor(Cambrex, Walkersville, Md.). Twenty-two milliliters of human marrow wasaseptically aspirated into a 25 ml syringe containing three millilitersof heparinized (1000 units per milliliter) saline solution. Theheparinized marrow solution was shipped overnight on ice to thelaboratory for bone marrow stromal cells isolation and culture. Uponarrival from the vendor, the twenty-five milliliter aspirates wereresuspended in Dulbecco's Modified Eagle Medium (DMEM) supplemented with10% fetal bovine serum (FBS), 0.1 mM nonessential amino acids, 100 U/mlpenicillin, 100 mg/L streptomycin (P/S), and 1 ng/ml basic fibroblastgrowth factor (bFGF) (Life Technologies, Rockville, Md.) and plated at8-10 microliters of aspirate/cm2 in tissue culture flasks. Fresh mediumwas added to the marrow aspirates twice a week for up to nine days ofculture. BMSCs were selected based on their ability to adhere to thetissue culture plastic; non-adherent hematopoietic cells were removedduring medium replacement after 9-12 days in culture. Medium was changedtwice per week thereafter. When primary BMSC became near confluent(12-14 days), they were detached using 0.25% trypsin/1 mM EDTA andreplated at 5×103 cells/cm². First passage (P1) hBMSCs were trypsinizedand frozen in 8% DMSO/10% FBS/DMEM for future use.

Frozen P1 hBMSCs were defrosted, replated at 5×103 cells/cm² (P2),trypsinized when near confluency, and used for fiber construct seeding.Sterilized (ethylene oxide) silk matrices (specifically, single cords ofMatrices 1 and 2, bundles of 30 parallel extracted silk fibers, andhelical ropes of collage fibers) were seeded with cells in customizedseeding chambers (1 ml total volume) machined in Teflon blocks tominimize cell-medium volume and increase cell-fiber construct contact.Seeded matrices, following a 4 hour incubation period with the cellslurry (3.3×106 BMSCs/ml) were transferred into a petri dish containingan appropriate amount of cell culture medium for the duration of theexperiments.

To determine the degradation rate of the silk fibroin, ultimate tensilestrength (UTS) was measured as a function of cultivation period inphysiological growth conditions, i.e., in cell culture medium. Groups of30 parallel silk fibers 3 cm in length were extracted, seeded withhBMSCs, and cultured on the fibroin over 21 days at 37° C. and 5% CO₂.Non-seeded control groups were cultured in parallel. Silk fibroin UTSwas determined as a function of culture duration for seeded andnon-seeded groups.

The response of bone marrow stromal cells to the silk fiber constructwas also examined.

BMSCs readily attached and grew on the silk and collagen matrices after1 day in culture (See FIG. 7A-C and FIG. 16A), and formed cellularextensions to bridge neighboring fibers. As shown in FIG. 7D and FIG.16B, a uniform cells sheet covering the construct was observed at 14 and21 days of culture, respectively. MTT analysis confirmed complete fiberconstruct coverage by seeded BMSCs after 14 days in culture (see FIG.8A-B). Total DNA quantification of cells grown on Matrix 1 (see FIG. 9A)and Matrix 2 (see FIG. 9B) confirmed that BMSCs proliferated and grew onthe silk construct with the highest amount of DNA measured after 21 and14 days, respectively, in culture.

Both BMSC-seeded or non-seeded extracted control silk fibroin groups of30 fibers, maintained their mechanical integrity as a function ofculture period over 21 days (see FIG. 10).

RT-PCR analysis of BMSCs seeded on cords of Matrix 2 indicated that bothcollagen I & III were upregulated over 14 days in culture (FIG. 14).Collagen type II and bone sialoprotein (as indicators of cartilage andbone specific differentiation, respectively) were either not detectableor negligibly expressed over the cultivation period. Real-timequantitative RT-PCR at 14 days yielded a transcript ratio of collagen Ito collagen III, normalized to GAPDH, of 8.9:1 (see FIG. 17). The highratio of collagen I to collagen III indicates that the response is notwound healing or scar tissue formation (as is observed with high levelsof collagen type III), but rather ligament specific; the relative ratioof collagen I to collagen III in a native ACL is ˜6.6:1 (Amiel et al.,Knee Ligaments: Structure, Function, Injury, and Repair, 1990).

Additionally, studies are conducted to provide insight into theinfluence of directed multi-dimensional mechanical stimulation onligament formation from bone marrow stromal cells in the bioreactorsystem. The bioreactor is capable of applying independent but concurrentcyclic multi-dimensional strains (e.g., translation, rotation) to thedeveloping ligaments. After a 7 to 14 day static rest period (time postseeding), the rotational and translation strain rates and linear androtational deformation are kept constant for 1 to 4 weeks. Translationalstrain (3.3%-10%, 1-3 mm) and rotational strain (25%, 90°) areconcurrently applied at a frequency of 0.0167 Hz (one full cycle ofstress and relaxation per minute) to the silk-based matrices seeded withBMSCs; an otherwise identical set of bioreactors with seeded matriceswithout mechanical loading serve as controls. The ligaments are exposedto the constant cyclic strains for the duration of the experiment days.

Following the culture period, ligament samples, both the mechanicallychallenged as well as the controls (static) are characterized for: (1)general histomorphological appearance (by visual inspection); (2) celldistribution (image processing of histological and MTT stainedsections); (3) cell morphology and orientation (histological analysis);and (4) the production of tissue specific markers (RT-PCR,immunostaining).

Mechanical stimulation markedly affects the morphology and organizationof the BMSCs and newly developed extracellular fiber construct, thedistribution of cells along the fiber construct, and the upregulation ofa ligament-specific differentiation cascade; BMSCs align along the longaxis of the fiber, take on a spheroid morphology similar toligament/tendon fibroblasts and upregulate ligament/tendon specificmarkers. Newly formed extracellular fiber construct (i.e., thecomposition of proteins produced by the cells) is expected to alignalong the lines of load as well as the long axis of the fiber construct.Directed mechanical stimulation is expected to enhance ligamentdevelopment and formation in vitro in a bioreactor resulting from BMSCsseeded on the novel silk-based fiber construct. The longitudinalorientation of cells and newly formed fiber construct is similar toligament fibroblasts found within an ACL in vivo (Woods et al., Amer. J.Sports Med. 19: 48-55, 1991). Furthermore, mechanical stimulationmaintains the correct expression ratio between collagen type Itranscripts and collagen type III transcripts (e.g., greater than 7:1)indicating the presence of newly formed ligament tissue versus scartissue formation. The above results will indicate that the mechanicalapparatus and bioreactor system provide a suitable environment (e.g.,multi-dimensional strains) for in vitro formation of tissue engineeredligaments starting from bone marrow stromal cells and the novelsilk-based fiber construct.

The culture conditions used in these preliminary experiments can befurther expanded to more accurately reflect the physiologicalenvironment of a ligament (e.g., increasing the different types ofmechanical forces) for the in vitro creation of functional equivalentsof native ACL for potential clinical use. These methods are not limitedto the generation of a bioengineered ACL. By applying the appropriatemagnitude and variety of forces experienced in vivo, any type ofligament in the body as well as other types of tissue can be produced exvivo by the methods of this disclosure.

Other embodiments are within the following claims.

TABLE 1 Ultimate tensile strength and stiffness (N/mm given a 3 cm longsample) as a function of sericin extraction from a 10-fiber silkwormsilk yarn with 0 twists per inch (i.e., parallel) and (i) temperatureand (ii) time. Repeat samples were processed two years after initialsamples with no significant change in properties. N = 5 for all samples.Stiffness/ # of UTS Stiff UTS/ fiber Yarn fibers Temp Time (N) stdev(N/mm) stdev fiber (N) (N/mm) 10(0) 10 RT 60 min 10.74 0.83 6.77 0.651.07 0.68 10(0) 10 RT 60 min (repeat) 10.83 0.28 6.36 0.14 1.08 0.6410(0) 10 33 C. 60 min 10.44 0.17 6.68 0.55 1.04 0.67 10(0) 10 37 C. 60min 9.60 0.84 6.09 0.59 0.96 0.61 10(0) 10 37 C. 60 min (repeat) 9.540.74 5.81 0.67 0.95 0.58 10(0) 10 90 C. 15 min 9.22 0.55 4.87 0.62 0.920.49 10(0) 10 90 C. 30 min 8.29 0.19 4.91 0.33 0.83 0.49 10(0) 10 90 C.60 min 8.60 0.61 4.04 0.87 0.86 0.40 10(0) 10 90 C. 60 min (repeat) 8.650.67 4.55 0.69 0.87 0.46 10(0) 10 94 C. 60 min 7.92 0.51 2.42 0.33 0.790.24 9(12s) × 3(9z) 27 non-extracted 24.50 0.38 8.00 0.49 0.91 0.309(12s) × 3(9z) 27 90 C. 60 min 21.88 0.18 7.38 0.34 0.81 0.27 9(6s) ×3(3z) 27 non-extracted 24.94 0.57 9.51 0.57 0.92 0.35 9(6s) × 3(3z) 2790 C. 60 min 21.36 0.40 7.95 1.00 0.79 0.29 9(12s) × 3(6z) 27non-extracted 24.69 0.65 9.08 0.56 0.91 0.34 9(12s) × 3(6z) 27 90 C. 60min 21.80 0.47 7.48 0.97 0.81 0.28

TABLE 2 Mass loss as a function of sericin extraction. +/−0.43% standarddeviation of an N = 5, reflects the greatest accuracy that can beachieved when confirming sericin removal, i.e., 0.87 or 1% error willalways be inherent to the methods used and a mass loss of about 24%represents substantially sericin free constructs. non-extractedextracted and % mass yarn and dried (mg) dried (mg) loss 9(12) × 3(6)57.6 43.6 24.31 9(12) × 3(6) 58.3 43.9 24.70 9(12) × 3(6) 57.0 42.924.74 9(12) × 3(6) 57.2 42.7 25.35 average 57.53 43.28 24.77 stdev 0.570.57 0.43

TABLE 3 Illustrates the change in mass as a function of a second sericinextraction. Correlated to FIG. 1E-1G, less than a 3% mass loss is likelyindicative of fibroin mass loss due to mechanical damage during the2^(nd) extraction. mass after 1x mass after 2x extraction, extracted, %mass yarn dried (mg) dried (mg) loss 9(12) × 3(6) 42.5 41.7 1.88 9(12) ×3(6) 43.1 42 2.55 9(12) × 3(6) 43.1 42.1 2.32 9(12) × 3(6) 42.5 41.71.88 9(12) × 3(6) 42.6 42.4 0.47 9(12) × 3(6) 43.7 42.4 2.97 9(12) ×3(6) 43.4 42.9 1.15 9(12) × 3(6) 43.7 43.1 1.37 9(12) × 3(6) 44 43.21.82 average 43.18 42.39 1.82 stdev 0.56 0.57 0.76

TABLE 4 # of Total UTS UTS Stiffness Stiffness UTS Stiff- Ply levels of# of average stdev % Elong % Elong avg stdev per ness per GeometryMethod Condition plying Fibers (N) (N) average stdev (N/mm) (N/mm) fiberfiber 1(0) × 3(10) cable extracted 2 3 1.98 0.05 10.42 1.63 2.17 0.510.66 0.72 1(0) × 4(10) cable extracted 2 4 2.86 0.14 11.98 1.54 2.080.31 0.72 0.52 3(0) × 3(3) cable extracted 2 9 6.72 0.17 12.30 0.72 4.540.16 0.75 0.50 1(0) × 3(10) × cable extracted 3 9 6.86 0.23 13.11 1.454.06 0.36 0.76 0.45 3(9) 2(0) × 6(11) cable 2 12 7.97 0.26 10.05 0.910.66 4(6) × 3(3) twist non- 2 12 10.17 0.18 19.86 1.16 0.85 extracted1(0) × 3(10) × cable extracted 3 12 9.29 0.19 14.07 0.98 5.10 0.31 0.770.43 4(9) 1(0) × 4(11) × twist extracted 3 12 9.70 0.14 12.56 1.03 7.600.33 0.81 0.63 3(11) 1(0) × 4(10) × cable extracted 3 12 8.78 0.17 14.251.09 5.10 0.32 0.73 0.43 3(9) 15 (textured) textured non- 1 15 10.620.68 10.76 1.70 4.75 0.30 0.71 0.316 extracted, dry 30(0) parallelextracted, 1 30 20.24 1.46 26.32 3.51 1.14 0.15 0.67 0.038 wet 30(0)parallel incubated 1 30 19.73 2.10 20.70 6.03 0.66 21 days, wet 30(0)parallel cell-seeded 1 30 20.53 1.02 29.68 7.08 0.68 21 days, wet 2fibers/carrier braid extracted, 2 16 10.93 0.13 6.96 1.14 0.68 0.435 inan 8 dry 4 fibers/carrier braid extracted, 2 32 24.60 0.22 12.39 0.530.77 0.387 in an 8 dry 4(6) × 3(3) in braid extracted, 3 48 37.67 0.1822.38 0.98 0.78 4 carrier dry 15(0) × 3(12) cable dry 2 45 27.39 0.6231.68 1.35 4.63 0.49 0.61 0.102889 15(0) × 3(12) × cable non- 3 13573.61 6.00 33.72 5.67 12.33 1.53 0.55 0.091333 3(10) extracted, 1 dayafter manufacturing 15(0) × 3(12) × cable non- 3 135 72.30 5.68 31.184.35 0.54 3(10) extracted, 2 days after manufacturing 15(0) × 3(12) ×cable non- 3 135 70.74 2.97 29.50 4.47 0.52 3(10) extracted, 3 daysafter manufacturing 15(0) × 3(12) × cable non- 3 135 75.90 1.57 34.574.12 0.56 3(10) extracted, 4 day after manufacturing 15(0) × 3(12) ×cable non- 3 135 71.91 5.71 36.72 3.75 0.53 3(10) extracted, 5 daysafter manufacturing 15(0) × 3(12) × cable non- 3 135 74.57 1.45 37.674.27 0.55 3(10) extracted, 6 days after manufacturing 13(0) × 3(11) ×cable non- 4 351 189.01 14.00 45.87 3.72 0.54 3(10) × 3(0) extracted,dry 13(0) × 3(11) × cable non- 4 351 170.12 7.37 39.95 1.37 0.48 3(10) ×3(0) extracted, cycled 30x to pretension, dry

TABLE 5 Comparison of UTS and stiffness between wet (2 hr incubation inPBS at 37° C.) and dry mechanical testing conditions. N = 5. Resultsshow approximately a 17% drop in UTS as a function of testing wet.Stiffness/ # of Yarn Test UTS UTS Stiffness Stiffness UTS/ fiber YarnFibers Conditions (N) stdev (N/mm) stdev fiber (N) (N/mm) 9(12s) × 3(9z)27 extracted - dry 21.88 0.18 7.38 0.34 0.81 0.27 9(12s) × 3(9z) 27extracted - wet 18.52 0.25 2.56 0.31 0.69 0.09 9(6s) × 3(3z) 27extracted - dry 21.36 0.40 7.95 1.00 0.79 0.29 9(6s) × 3(3z) 27extracted - wet 17.94 0.30 2.40 0.28 0.66 0.09 9(12s) × 3(6z) 27extracted - dry 21.80 0.47 7.48 0.97 0.81 0.28 9(12s) × 3(6z) 27extracted - wet 18.74 0.22 2.57 0.11 0.69 0.10 12(0) × 3(10s) 36extracted - dry 30.73 0.46 16.24 0.66 0.85 0.45 12(0) × 3(10s) 36extracted - wet 25.93 0.29 6.68 0.70 0.72 0.19 4(0) × 3(10s) × 3(9z) 36extracted - dry 30.07 0.35 15.49 1.06 0.84 0.43 4(0) × 3(10s) × 3(9z) 36extracted - wet 22.55 0.66 7.63 1.00 0.63 0.21

TABLE 6 Effect of TPI on UTS and Stiffness. N = 5 Stiff- UTS/ Stiffness/UTS ness fiber fiber Yarn TPI (N) stdev (N/mm) stdev (N) (N/mm) 12(0) ×3(2) 2 23.27 0.28 6.86 0.60 0.65 0.19 12(0) × 3(4) 4 24.69 0.31 7.611.17 0.69 0.21 12(0) × 3(6) 6 25.44 0.42 6.51 1.35 0.71 0.18 12(0) ×3(8) 8 25.21 0.23 5.80 0.67 0.70 0.16 12(0) × 3(10) 10 25.94 0.24 6.450.77 0.72 0.18 12(0) × 3(12) 12 25.87 0.19 6.01 0.69 0.72 0.17 12(0) ×3(14) 14 22.21 0.58 5.63 0.71 0.62 0.16

TABLE 7 Additional tpi data to verify that up to 30 tpi can be usedwithout causing damage to the yarn that would result in a dramaticdecrease in UTS and stiffness; note, all matrices (N = 5 per group) weretwisted. Stiffness/ # of UTS stdev Stiffness stdev UTS/ fiber Yarnfibers (N) (N) (N/mm) (N/mm) fiber (N) (N/mm) Conditions 1(30) × 6(20) ×3(4.5) 18 10.92 0.44 1.21 0.02 0.61 0.07 non-extracted, wet 1(30) ×6(20) × 3(10) 18 11.48 0.37 1.25 0.06 0.64 0.07 non-extracted, wet 1(30)× 6(6) 6 3.83 0.24 0.37 0.04 0.64 0.06 non-extracted, wet 15(20) 1513.19 0.27 6.03 0.67 0.88 0.40 extracted, dry

TABLE 8 Effect of yarn hierarchy on mechanical properties (i.e. thenumber of levels and the number of fibers per level can significantlyinfluence yarn and fabric outcomes. # of Stiffness Stiffness UTS Stiff-levels of Total # UTS UTS % Elong % Elong avg stdev per ness perGeometry Condition plying of Fibers (N) stdev (N) average stdev (N/mm)(N/mm) fiber fiber 1(0) × 3(10) extracted 2 3 1.98 0.05 10.42 1.63 2.170.51 0.66 0.72 1(0) × 3(10) × 3(9) extracted 3 9 6.86 0.23 13.11 1.454.06 0.36 0.76 0.45 1(0) × 3(10) × 4(9) extracted 3 12 9.29 0.19 14.070.98 5.10 0.31 0.77 0.43 1(0) × 4(10) extracted 2 4 2.86 0.14 11.98 1.542.08 0.31 0.72 0.52 1(0) × 4(10) × 3(9) extracted 3 12 8.78 0.17 14.251.09 5.10 0.32 0.73 0.43 15(0) × 3(12) non- 2 45 27.39 0.62 31.68 1.354.63 0.49 0.61 0.10 extracted dry 15(0) × 3(12) × 3(10) non- 3 135 73.616.00 33.72 5.67 12.33 1.53 0.55 0.09 extracted dry

TABLE 9 Surface modification (RGD and ETO gas sterilization) effects onextracted silk matrix mechanical properties; PBS was used as a negativecontrol during modification treatments. Surface UTS/ Stiffness/ # ofModification/ UTS Stiffness fiber fiber Yarn fibers Sterilization (N)stdev (N/mm) stdev (N) (N/mm) 12(0) × 3(10s) 36 Non-treated 25.94 0.246.45 0.77 0.72 0.18 12(0) × 3(10s) 36 RGD 23.82 2.10 3.79 2.06 0.66 0.1112(0) × 3(10s) × 3(9z) 108 Non-treated 48.89 4.84 9.22 0.84 0.45 0.0912(0) × 3(10s) × 3(9z) 108 RGD 55.28 3.28 8.17 0.81 0.51 0.08 4(11s) ×3(11z) × 3(10s) 36 ETO 18.72 0.45 5.52 0.42 0.52 0.15 4(11s) × 3(11z) ×3(10s) 36 RGD + ETO 19.30 0.62 4.67 0.3 0.54 0.13

TABLE 10 UTS Stiffness Yield Pt. Elongation (N) (N/mm) (N) (%) Silk 2337+/− 72 354 +/− 26 1262 +/− 36  38.6 +/− 2.4 matrix 1 Silk 3407 +/− 63580 +/− 40 1647 +/− 214 29 +/− 4 Matrix 2 Human  2160 +/− 157 242 +/− 28~1200 ~26-32% ACL Mechanical properties for two different cords based ona cord length of 3 cm as compared to human ACL properties.

TABLE 11 Twisting Level (# of twists/cm) Matrix 1 Matrix 2 Matrix 3Matrix 4 Matrix 5 Matrix 6 Matrix 7 # fibers per group 30 (0)  15 (0) 1300 (0)   180 (0)  20 (0)  10 (0)  15 (0)  # groups per bundle 6 (3) 3(3) 3 (2)   3 (3.5) 6 (3) 6 (3) 3 (3) # bundles per strand 3 (3)   6(2.5) 1 (0) 3 (2) 3 (2)   3 (2.5)   3 (2.5) # strands per cord 6 (0)   3(2.0) — 2 (0) 3 (1) 3 (2) 3 (2) # cords per ACL — 6 (0) — — 3 (0) 6 (0)12 (0)  UTS (N) 2337 3407 2780 2300 2500 2300 3400 Stiffness (N/mm)  354 580  300  350  550  500  550 Examples of several geometry hierarchiesthat would result in suitable mechanical properties for replacement ofthe ACL. Note: Matrix 1 and 2 have been developed as shown in theexamples; Matrix 3 would yield a single bundle prosthesis, Matrix 4would yield a 2 strand prosthesis, Matrix 5 would yield a 3 cordprosthesis, Matrix 6 is another variation of a 6 cord prosthesis, andMatrix 7 will yield a 12 cord prosthesis.

We claim:
 1. A method for facilitating tissue generation comprising thestep of implanting a biodegradable silk fabric into the individual,wherein: (a) the biodegradable silk fabric is comprised of one or moreindividual yarns comprised of silkworm derived, sericin-extracted silkfibroin fibers, and; (b) the silk fibroin fibers have not been dissolvedand reconstituted.
 2. The method of claim 1, wherein the fabric isimplanted intramuscularly.
 3. The method of claim 1, wherein the fabricis implanted into an individual to replace or repair damaged tissue. 4.The method of claim 1, wherein the fabric is implanted subcutaneously.5. The method of claim 1, wherein the fabric has one or morebiomechanical properties of a connective tissue.
 6. A method forfacilitating tissue generation comprising the step of subcutaneouslyimplanting in an individual a biodegradable silk fabric that has one ormore biomechanical properties of a connective tissue into theindividual, wherein: (a) the biodegradable silk fabric is comprised ofone or more individual yarns comprised of silkworm derived, sericinextracted silk fibroin fibers, and; (b) the silk fibroin fibers have notbeen dissolved and reconstituted, and wherein the fabric is implantedinto the individual to replace or repair damaged tissue.